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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