Support extensions

[judah]

Extensions (proto2-only) currently aren't supported yet. It's not clear what the API should look like.

This would primarily be useful for legacy code, since proto3 replaces extensions with the Any type (#22).

[gleber]

Judah, have you given any thoughts about this feature?

[judah]

I haven't really had a chance to consider it in detail. Here's a potential approach which is along the same lines as other language bindings:

First, define an analogue RawMessage of google::protobuf::Message, which essentially holds the raw tagged values in a Map.

Now, suppose we have the following code in foo.proto:

// File foo.proto
message Foo {
    extensions 100 to 199;
}

We add a new field to Foo with name _Foo__extensions and type RawMessage. (This approach would also essentially solve #29.)

Finally, suppose we have in bar.proto:

extend Foo {
   optional int32 bar = 123;
}

message Baz {
    extend Foo {
        optional Baz foo_ext = 124;
    }
}

Then we define two new instances of Lens.Label.HasLens: HasLens "bar" f Foo Foo Int32 Int32, and HasLens "foo_ext" f Foo Foo Baz Baz; where the implementation of each lens uses the RawMessage that Foo holds.