Add support for recursively defined message structures.

[EdSchouten]

I'm trying to use this library in combination with a relatively complex
protocol, namely the remote execution protocol of the Bazel build sytem:

https://github.com/bazelbuild/remote-apis/blob/master/build/bazel/remote/execution/v2/remote_execution.proto

Some of the dependencies of this protocol contain recursively defined
messages. These currently cause the compiler to emit Elm code that isn't
valid, due to recursive alias types.

Solve this by adding a "%sMessage" custom type for every message. This
causes the recursion to be broken. Let all nested messages use this type
and fix up the encode/decode functions to properly destruct/pack
messages into the custom type.

[EdSchouten]

Here are the messages that contain recursive declarations:

https://github.com/googleapis/googleapis/blob/master/google/api/http.proto <- HttpRule
https://github.com/protocolbuffers/protobuf/blob/master/src/google/protobuf/descriptor.proto <- DescriptorProto

[EdSchouten]

(Friendly ping?) :-)

[tiziano88]

Thanks for the PR! I was wondering, would it make sense to just avoid the type alias, and define a new (inlined) type for each message?

e.g.

current:

type alias Bar =
    { field : Bool -- 1
    }
type BarMessage = BarMessage Bar

proposed:

type Bar = Bar
    { field : Bool -- 1
    }

Then we only use the new type everywhere, even though it requires always explicitly referring to the constructor (both for creating new instances, and also for pattern matching).

WDYT?