vdl: Add codegen for Any, and fix some VDLRead bugs.

After this CL, if you manually replace vom.Decoder with
vom.XDecoder, all tests pass, except for a single failure
involving a nil vom.RawBytes.

MultiPart: 1/2
Change-Id: I6d96d7e6f655e859dde9dc2189f621fe6f125866
16 files changed