Better support for maps

Hello,
While testing a bit this library in our code base, I noted that map support with pool is not perfect.

• map themselves are not pooled
• ResetVT does not return to pool values from a map<key, MessagePooled>
• UnmarshalVT allocates a new message instead of using the pool for map<key, MessagePooled>

Doing these changes manually in the .pb.go reduces a lot allocations and speeds up unmarshalling.

I can provide a sample proto file and modifications made if needed.

Pre changes:
BenchmarkUnmarshalStdProto-12 202058 4971 ns/op 3840 B/op 54 allocs/op
BenchmarkUnmarshalVTProto-12 228591 5238 ns/op 3429 B/op 47 allocs/op
BenchmarkUnmarshalVTProtoWithPool-12 238689 4967 ns/op 2605 B/op 44 allocs/op

Post changes:
BenchmarkUnmarshalStdProto-12 203602 5240 ns/op 3840 B/op 54 allocs/op
BenchmarkUnmarshalVTProto-12 199917 5864 ns/op 3433 B/op 47 allocs/op
BenchmarkUnmarshalVTProtoWithPool-12 601562 2009 ns/op 302 B/op 5 allocs/op

The issue does not name files or tests; begin by locating the generator code responsible for map handling in generated .pb.go files and review the existing pool paths. Reproduce the supplied benchmarks with map<key, MessagePooled> fields, then verify that maps and their values are pooled by ResetVT and UnmarshalVT without regressing behavior.