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#`.