parseFrom methods return instancetype

This makes the parse code more flexible and boilerplate.  Possible move
into the core library and / or protocol ? There may be a foundation
protocol that can replace these.
runtime library adopts instancetype
messages and builders return instancetype

Currently these classes are intermingled but this is already showing
code that can move down into the library from the generator.
messages and builders implement instancetype
Merge branch 'gen-instancetype'