Reworks original PR #591 by @Iliya-usov

Original improvements and fixes:

• Implement Memory.IsReadWriteAtomic
• Make maybe return consistent value with hasValue
• Fix tearing issues with myValue if T is long struct
• Enforce memory ordering between myValue = value and myChange.Fire(value);

This PR reworks Memory.IsReadWriteAtomic to be consistent with CLR memory model and replaces corresponding test harness accordingly. No need for runtime sizeof support required - so no dependency on Unsafe/ReflectionEmit.

FYI @ForNeVeR