plugins: Fix code generation for a conflicting oneof

[sigaev]

To reproduce the problem that this pull request fixes:

1. Create file x.proto with the following contents:

syntax = "proto3";
message M {
  oneof b {
    bool a = 1;
    bool get_b = 2;  // Conflicts with "oneof b".
  }
}

2. Run the following commands:

go get github.com/gogo/protobuf/protoc-gen-gofast
protoc --gofast_out=. --plugin=$HOME/go/bin/protoc-gen-gofast x.proto
grep M_GetB x.pb.go

3. Output will show that MarshalTo, MarshalToSizedBuffer, Size
   all have an incorrectly named receiver, M_GetB. It needs to be
   M_GetB_ instead:

	//	*M_GetB_
type M_GetB_ struct {
func (*M_GetB_) isM_B() {}
	if x, ok := m.GetB().(*M_GetB_); ok {
		(*M_GetB_)(nil),
func (m *M_GetB) MarshalTo(dAtA []byte) (int, error) {
func (m *M_GetB) MarshalToSizedBuffer(dAtA []byte) (int, error) {
func (m *M_GetB) Size() (n int) {
			m.B = &M_GetB{b}

[jo-asakura]

Hi @jmarais! Could you help with the PR? It's been an issue for several projects on our side as well.