Tracking issue for `thread_local_state` stabilization

[aturon]

This feature allows you to query a thread-local variable for its state: http://static.rust-lang.org/doc/master/std/thread/enum.LocalKeyState.html, which will then tell you what may happen if you try to read from the variable.

The functionality is somewhat niche, but occasionally essential. The APIs appear to be in pretty decent shape as-is.

cc @alexcrichton

[aturon]

Nominating for stabilization in 1.6.

[alexcrichton]

The libs team decided to not move this into FCP, some worries around this being:

• Checking the state and then accessing hits TLS twice, which may be too slow for some use cases.
• This is very similar to RefCell::borrow_state which has also not yet been stabilized. It's likely that the enum here wants to move to boolean accessors so we can add more states in the future.

[alexcrichton]

Although I nominated this for 1.8, libs decided to remove the nomination as I forgot that the reason this is not in FCP yet is because you hit TLS twice when querying the state.

[apasel422]

I think this would be better expressed like RefCell::try_borrow.

[PlasmaPower]

See also: rust-lang/rfcs#2030

[PlasmaPower]

PR for try_with (similar to RefCell::try_borrow) wrapped into this feature: #43158