Implement MutatorContextCheck on Windows. Add AVERT(MutatorContext, context) in PoolAccess and other functions.
Document initialization functions MutatorContextInitFault and MutatorContextInitThread and implement them on Windows.
Add a union discriminator to MutatorContextStruct on Windows so that we don't accidentally try to get the stack pointer from a fault context, or the exception address from a thread context.
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Change: 192567
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This means that ThreadScan becomes identical on these two platforms and can be moved to thw3.c.
This means that thw3.h, thw3i3.c and thw3i6.c become redundant and can be deleted.
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Change: 192534
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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.
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Change: 192528
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Rename prmc{i3,i6}{fr,li,w3,xc}.c to prmc{fr,li,w3,xc}{i3,i6}.c for consistency of ordering of platform codes (OS before AR before CT).
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Change: 192512
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