Add an EqualMessageVT generator

[misberner]

This PR adds a generator that makes EqualVT accessible with a uniform signature as EqualMessageVT(proto.Message) bool.

The main motivation is to ease migration from Gogo protobuf by offering a drop-in replacement for "github.com/gogo/protobuf/proto".Equal, which uses optimized code where available. With the change in this PR, this can simply be done via

func Equal(a, b proto.Message) bool {
  if aVT, ok := a.(interface{ EqualMessageVT(proto.Message) bool }); ok {
    return aVT.EqualMessageVT(b)
  }
  // we can disregard checking b because if a and b are not of the same type, Equal will be fast anyway
  return proto.Equal(a, b) // from google.golang.org/protobuf/proto
}

I had added a similar facility when adding CloneVT via CloneGenericVT. However, with the launch of generics in go1.18, this naming has not aged well. I therefore adopted ...Message... for the variant accepting a proto.Message, and have also applied this by renaming CloneGenericVT to CloneMessageVT (the old variant is still generated for backwards-compatibility, but marked deprecated).

[vmg]

Looks good! Thanks for your contribution! I'm going to push a separate change to remove the deprecation notice from the CloneGenericVT API: since we've never actually released a version of vtprotobuf with that API, we can rename it without having to keep the old version around indefinitely.