crucible-mir: Add Mir.TransCustom overrides for {read,write}_volatile

[RyanGlScott]

The read_volatile and write_volatile Rust functions are useful when dealing with code that involves memory sanitization. Currently, crucible-mir lacks any Mir.TransCustom support for these functions, so we should add them. The most straightforward way to do so would be to treat them like ordinary reads and writes. That is:

volatile_load :: (ExplodedDefId, CustomRHS)
volatile_load = ( ["core", "intrinsics", "{extern}", "volatile_load"], \substs -> case substs of
    Substs [_] -> Just $ CustomOp $ \_ ops -> case ops of
        [MirExp (MirReferenceRepr tpr) ptr] ->
            MirExp tpr <$> readMirRef tpr ptr
        _ -> mirFail $ "bad arguments for ptr::read: " ++ show ops
    _ -> Nothing)

volatile_store :: (ExplodedDefId, CustomRHS)
volatile_store = ( ["core", "intrinsics", "{extern}", "volatile_store"], \substs -> case substs of
    Substs [_] -> Just $ CustomOp $ \_ ops -> case ops of
        [MirExp (MirReferenceRepr tpr) ptr, MirExp tpr' val]
          | Just Refl <- testEquality tpr tpr' -> do
            writeMirRef ptr val
            return $ MirExp C.UnitRepr $ R.App E.EmptyApp
        _ -> mirFail $ "bad arguments for ptr::write: " ++ show ops
    _ -> Nothing)