add support for enum in a message

[purtato]

I noticed that nested messages don't seem to be supported #1

So for now i've changed the goal of this PR to make it such that any .proto prior to #63 that was previously working, should still continue to work

The above caveat is explicit because while

message A {
  message B {
    enum C {
      ....
    }
  }
}

will not work, it should at the very least not make things worse

[purtato]

also should note that

message A {
  enum C{}
}

message B {
  enum C{}
}

will cause a conflict, prepending the enum name with the message name could make this less likely but isn't guarenteed conflict free
e.g.

message AB {
  enum C{}
}

message A {
  enum BC{}
}