Decoding choice elements that can hold empty structs (#120)

## Introduction

In merging in #119, we fixed most but not quite all of #91! Decoding of _null_ choice elements (represented as enums with empty struct associated values on the Swift side) still results in errors. This PR adds a test to demonstrate and fixes this issue by wrapping each `NullBox` inside of a `SingleKeyedBox` at the `merge` phase (called by `transformToBoxTree`).

## Motivation

One of the main lessons from #119 was that we have to wrap choice elements in the decoding phase to hold onto their keys. The keys are needed for directing us to the correct branch of the do-catch pyramid used for decoding. 

```swift
private enum Entry: Equatable {
    case run(Run)
    case properties(Properties)
    case br(Break)
}

extension Entry: Decodable {
    private enum CodingKeys: String, XMLChoiceCodingKey {
        case run, properties, br
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        do {
            self = .run(try container.decode(Run.self, forKey: .run))
        } catch {
            do {
                self = .properties(try container.decode(Properties.self, forKey: .properties))
            } catch {
                self = .br(try container.decode(Break.self, forKey: .br))
            }
        }
    }
}
```
where one of the associated values could be an empty struct (represented by null):

```swift
private struct Break: Decodable {}
```

Although we _can_ throw out keys for non-choice null elements, a mechanism is needed for holding onto the keys while transforming from the `XMLCoderElement` tree to the `boxTree`. Only later will we know if the key is needed (if this wrapped element is transformed to a `ChoiceBox`); if not, we will be able to throw out the key. 

## Proposed solution

The Public API is unchanged. On the implementation side, we catch `NullBox` values in `merge` and wrap them in `SingleKeyedBox` instances. 

## Detailed Design

In `merge`, we wrap each `NullBox` in a `SingleKeyedBox` with the appropriate key bundled in. An `XMLChoiceDecodingContainer` can be constructed from the `SingleKeyedBox` by converting it to a `ChoiceBox` (just transferring over the contents) - as normal. In `XMLKeyedDecodingContainer`, when preparing the `elements` for concrete decoding, we unwrap all `SingleKeyedBox` values that may be contained therein, as any choice elements contained would have already been transformed to a `ChoiceBox` by this point in decoding: any stray `SingleKeyedBox` wrappers can thus be thrown out. 

## Source compatibility

This is purely an additive change.

Sources/XMLCoder/Auxiliaries/KeyedStorage.swift

@@ -83,7 +83,7 @@ extension KeyedStorage where Key == String, Value == Box {
         } else if let value = element.value {
             result.append(StringBox(value), at: element.key)
         } else {
-            result.append(NullBox(), at: element.key)
+            result.append(SingleKeyedBox(key: element.key, element: NullBox()), at: element.key)
         }
 
         return result

Sources/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift

@@ -79,6 +79,10 @@ struct XMLKeyedDecodingContainer<K: CodingKey>: KeyedDecodingContainerProtocol {
 
         let box = elements.first ?? attributes.first
 
+        if let singleKeyed = box as? SingleKeyedBox {
+            return singleKeyed.element.isNull
+        }
+
         return box?.isNull ?? true
     }
 
@@ -232,14 +236,26 @@ extension XMLKeyedDecodingContainer {
                     let keyString = key.stringValue.isEmpty ? "value" : key.stringValue
                     let value = keyedBox.elements[keyString]
                     if !value.isEmpty {
-                        return value
+                        return value.map {
+                            if let singleKeyed = $0 as? SingleKeyedBox {
+                                return singleKeyed.element
+                            } else {
+                                return $0
+                            }
+                        }
                     } else if let value = keyedBox.value {
                         return [value]
                     } else {
                         return []
                     }
                 } else {
-                    return keyedBox.elements[key.stringValue]
+                    return keyedBox.elements[key.stringValue].map {
+                        if let singleKeyed = $0 as? SingleKeyedBox {
+                            return singleKeyed.element
+                        } else {
+                            return $0
+                        }
+                    }
                 }
             }
 

Tests/XMLCoderTests/NestedChoiceTests.swift

@@ -227,6 +227,50 @@ class NestedChoiceTests: XCTestCase {
         XCTAssertEqual(result, expected)
     }
 
+    func testNestedEnumsWithEmptyStruct() throws {
+        let xml = """
+        <container>
+            <p>
+                <br></br>
+                <run>
+                    <id>1518</id>
+                    <text>I am answering it again.</text>
+                </run>
+                <properties>
+                    <id>431</id>
+                    <title>A Word About Wake Times</title>
+                </properties>
+            </p>
+            <p>
+                <run>
+                    <id>1519</id>
+                    <text>I am answering it again.</text>
+                </run>
+                <br />
+            </p>
+        </container>
+        """
+        let result = try XMLDecoder().decode(Container.self, from: xml.data(using: .utf8)!)
+        let expected = Container(
+            paragraphs: [
+                Paragraph(
+                    entries: [
+                        .br(Break()),
+                        .run(Run(id: 1518, text: "I am answering it again.")),
+                        .properties(Properties(id: 431, title: "A Word About Wake Times")),
+                    ]
+                ),
+                Paragraph(
+                    entries: [
+                        .run(Run(id: 1519, text: "I am answering it again.")),
+                        .br(Break()),
+                    ]
+                ),
+            ]
+        )
+        XCTAssertEqual(result, expected)
+    }
+
     func testNestedEnumsRoundTrip() throws {
         let original = Container(
             paragraphs: [