Replace the atomicModifyMutVar# primop

This was accepted proposal #27. Specifically:

1. Replace atomicModifyMutVar# with atomicModifyMutVar2#.
2. Add atomicModifyMutVar_# and atomicSwapMutVar#.
3. Add a function imitating atomicModifyMutVar# to GHC.Exts for backwards compatibility.
4. Use the new primops where appropriate throughout base.

Trac metadata

Trac field	Value
Version	8.5
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
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Blocking
CC	simonmar
Operating system
Architecture