This crate was written by Amanieu d'Antras in response to my efforts to make
thread safe caching in the regex crate faster. The way to think about it is
"dynamic" thread local storage.

The crate does use a fair amount of unsafe, and the reason for this is to
minimize latency as much as is possible. Each use of unsafe isn't
particularly tricky---it's mostly managing atomic access to a hash table.
With that said, it would be nice if each use of unsafe had a comment
explaining/justifying it. There are some comments scattered about making
reasonable arguments, but more would be good.

This crate did have an issue a while back where if a program used the same
regex (for example) many times from many short lived threads, then the
cached data could grow without bound. Amanieu patched that issue a while
back by reusing thread ids. A thread id is tracked using normal TLS, which
is dropped when the thread is destroyed.

I gave this crate a rating of positive because I cannot identify any issues
after reviewing the code, although, I am not completely certain that there
are no issues because of the use of unsafe code.