Naming, value types and tests for Addr# atomics

The atomic Exchange and CAS operations on integral types are updated to
take and return more natural `Word#` rather than `Int#` values.  These
are bit-block not arithmetic operations, and the sign bit plays no
special role.

Standardises the names to `atomic<OpType><ValType>Addr#`, where `OpType` is one
of `Cas` or `Exchange` and `ValType` is presently either `Word` or `Addr`.
Eventually, variants for `Word32` and `Word64` can and should be added,
once #11953 and related issues (e.g. #13825) are resolved.

Adds tests for `Addr#` CAS that mirror existing tests for
`MutableByteArray#`.

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