Asynchronous threading phase2

[john-sharratt]

This patch does a series of refactoring to prevent similar bugs to the one that was found in the asynchronous threading patch.

In particular:

• inner() is scoped to the wasi module and no longer usable outside
• inner() is marked as unsafe and any use of it is audited and wrapped with unsafe
• Cloning WasiEnv no longer copies the inner()
• Rewinding the stack now passes a return value back to the rewound stack rather than using the memory (which is unsafe)
• The after callback on asynchronous threading has been removed and replaced with rewind return values

[theduke]

Why do we need that thread local?

If we really do need it, we can guard against bugs by storing some thread-relevant value in the thread local and panic on a mismatch.
(We can just store the whole WasiThread, since it's clone, and do Arc::ptr_eq for equality)

(rather than making the functions unsafe)

[theduke]

Why do we need that thread local?

If we really do need it, we can guard against bugs by storing some thread-relevant value in the thread local and panic on a mismatch.
(We can just store the whole WasiThread, since it's clone, and do Arc::ptr_eq for equality)

(rather than making the functions unsafe)

[theduke]

Much cleaner now without the thread locals!