Unflatten types

[joshcarp]

See: https://github.com/anz-bank/protoc-gen-sysl/blob/master/tests/messageinmessage/simple2.proto

Messages defined within messages will be squished

[joshcarp]

message this {
   enum whatever{
        this = 1;
        that = 2;
    }
}

gets flattened to this_whatever in sysl

[joshcarp]

!type this:
    !enum whatever:
        this: 1;
        that: 2;

instead of the current:

!enum this_whatever:
    this: 1;
    that: 2;