Code Generation for fast marshaling

[jcorbin]

Wanted: a code generator for zapcore.ObjectMarshaler and (by extension) zapcore.ArrayMarshaler

For those cases where you really do want to log your domain objects, but don't want to hit the reflection path, and don't want to take on the transitive implementation burden (say to cover all of the implied IDL structure of your domain object).

For obvious reasons, we may want to support a struct field tag to exempt some fields (i.e. zap:"dontlog" or something more generic).