codecgen: support named types in generated code properly.
codecgen: support named types in generated code properly.

When generating method names, use the same logic as for non-composite
types in the switch below.

Fixes #73
Fixes #68

sunvim/go,ugorji/go,sunvim/go,whyrusleeping/go-codec,whyrusleeping/go-codec,whyrusleeping/go-codec,ugorji/go,sunvim/go,ugorji/go