Rename functions Prot{Can,}StepInstruction to MutatorContext{Can,}StepInstruction so that they follow the naming convention in guide.implc.naming.prefix.program.
Move mutator context declarations out of prot.h into new header prmc.h.
Correct .assume.null in a couple of places -- it's not safe for MutatorContextStepInstruction to return ResUNIMPL, instead MutatorContextCanStepInstruction should return FALSE.
Copied from Perforce
Change: 192528
ServerID: perforce.ravenbrook.com
Enabling amcssth and awlutth tests for all platforms, since I believe these were fixed by OS X threads development.
Copied from Perforce
Change: 183118
ServerID: perforce.ravenbrook.com
