Implement EarlyMutators with BottomUpMutators
Implement EarlyMutators with BottomUpMutators

Get rid of runEarlyMutators, and implement EarlyMutators as a shim
around BottomUpMutators.  Removes mostly-duplicate code between
runEarlyMutators and runBottomUpMutator to allow parallelizing
runBottomUpMutator.

Change-Id: I66a051cb8710770b47da21d35e55aa49c84e9be8

google/blueprint,google/blueprint,danw/blueprint,danw/blueprint