Feature request: set() and clear() accessors return this

Also pre-inlines set() and has() in serialization code. This could
theoretically help ProGuard: the message class size is usually large,
and because of this only, it may refuse to inline an accessor into
the serialization code, and as a result keeps the accessor intact.
Chances are, after pre-inlining all accessor calls within the message
class, those accessors become unused or single-use, so there are more
reasons for ProGuard to inline and then remove them.

Change-Id: I57decbe0b2533c1be21439de0aad15f49c7024dd