xenon: move into its own directory, cleanups

Summary: - move into runtime/ext/xenon
- add 'noexcept', 'const' where appropriate
- migrate from 'static' to anonymous namespace
- final class xenon
- 'override' on inherited virtuals

Reviewed By: @jano

Differential Revision: D1581855
remove async stacks from xenon

Reviewed By: @jano

Differential Revision: D1582160