Simplify Encoder's encoding signature.

The reality is that if we ever try to encode a Protocol Buffer and it
fails, it's likely that such an error is ultimately not a runtime error
and should be fixed forthwith.  Thusly, we should rename
``Encoder.Encode`` to ``Encoder.MustEncode`` and drop the error return
value.
Destroy naked returns in half of corpus.

The use of naked return values is frowned upon.  This is the first
of two bulk updates to remove them.
Merge pull request #254 from prometheus/feature/refactor/serialization

Several Refactorings: Simplified Encoder / Reduction of Naked Return Values