Reduce memory overhead of sourcemapLocations

It was using an object to store positions in the string. This meant that
those offsets were first stringified and then added as object
properties. For a 200K JS code file (as parsed by rollup), the
sourcemapLocations object was taking up 980K. Switching it to a BitSet
that uses one bit per offset makes it take up 36K instead.

Fixes Rich-Harris#167

src/BitSet.js

@@ -0,0 +1,15 @@
+export default class BitSet {
+	constructor(arg) {
+		this.bits = arg instanceof BitSet ? arg.bits.slice() : [];
+	}
+
+	add(n) {
+		this.bits[Math.floor(n / BITS)] |= 1 << n % BITS;
+	}
+
+	has(n) {
+		return !!(this.bits[Math.floor(n / BITS)] & (1 << n % BITS));
+	}
+}
+
+const BITS = 32;

src/MagicString.js

@@ -1,3 +1,4 @@
+import BitSet from './BitSet.js';
 import Chunk from './Chunk.js';
 import SourceMap from './SourceMap.js';
 import guessIndent from './utils/guessIndent.js';
@@ -30,7 +31,7 @@ export default class MagicString {
 			byEnd:                 { writable: true, value: {} },
 			filename:              { writable: true, value: options.filename },
 			indentExclusionRanges: { writable: true, value: options.indentExclusionRanges },
-			sourcemapLocations:    { writable: true, value: {} },
+			sourcemapLocations:    { writable: true, value: new BitSet() },
 			storedNames:           { writable: true, value: {} },
 			indentStr:             { writable: true, value: guessIndent(string) }
 		});
@@ -44,7 +45,7 @@ export default class MagicString {
 	}
 
 	addSourcemapLocation(char) {
-		this.sourcemapLocations[char] = true;
+		this.sourcemapLocations.add(char);
 	}
 
 	append(content) {
@@ -121,9 +122,7 @@ export default class MagicString {
 			cloned.indentExclusionRanges = this.indentExclusionRanges.slice();
 		}
 
-		Object.keys(this.sourcemapLocations).forEach(loc => {
-			cloned.sourcemapLocations[loc] = true;
-		});
+		cloned.sourcemapLocations = new BitSet(this.sourcemapLocations);
 
 		cloned.intro = this.intro;
 		cloned.outro = this.outro;

src/utils/Mappings.js

@@ -28,7 +28,7 @@ export default class Mappings {
 		let first = true;
 
 		while (originalCharIndex < chunk.end) {
-			if (this.hires || first || sourcemapLocations[originalCharIndex]) {
+			if (this.hires || first || sourcemapLocations.has(originalCharIndex)) {
 				this.rawSegments.push([this.generatedCodeColumn, sourceIndex, loc.line, loc.column]);
 			}