sbe: Codec::New binds a message with a oneof field, which protowire-go refuses #141

This port binds an SBE message that has a real oneof field, and protowire-go refuses it. Found while porting #132 (PR #140).

Go (encoding/sbe/template.go, since the initial release d137dd0) refuses a field in a non-synthetic oneof:

• in a message: sbe: oneof field %s.%s not supported
• in a composite: composite %s contains oneof field %s (wrapped by the caller, for example sbe: composite M.c: composite C contains oneof field f)
• in a group entry: no check, so a oneof there binds in Go too.

A proto3 optional field sits in a synthetic oneof and is not refused.

C++ now (read from src/sbe/template.cc on the #140 branch, not measured): BuildTemplate and BuildCompositeFields have no oneof check, so such a message binds. Every member gets its own slot in the block, and Marshal writes all of them, unset members as zero.

Done when: Codec::New refuses a oneof member in a message and in a composite with Go's text, a proto3 optional field still binds, a group entry's oneof behaves as Go's does, and a test pins each case.

This narrows what binds, so the CHANGELOG should say so.