Add Iter

[Kestrer]

Now the values inside the thread-local can be accessed concurrently by other threads.

There is a slight performance impact; get now takes 2.7ns instead of 2.5, and insert takes 45ns instead of 42.

[Kestrer]

Experiementally I have added a check so that on rustc versions < 1.36.0 it uses an Option but on rustc versions >= 1.36.0 it uses MaybeUninit. Surprisingly performance is entirely unaffected, although that may just be because the benchmarks we ran were able to perform the null-pointer optimization before, and so there wasn't a cost to using an Option.

[BurntSushi]

I wouldn't pin it. It's a little bit of a pain, but users who still need to support such an old version of Rust can pin thread_local themselves in their Cargo.lock file.

I plan to bump the MSRV of regex to 1.41.1 soon anyway. The existing 1.28 MSRV is ancient. The only reason why it's still that ild is that I haven't had a chance to go through the upgrade process.

[Kestrer]

Ok, I've updated the MSRV to 1.36.0. This also allows us do some other nice things like update to 2018 edition, use once_cell instead of lazy_static, elide some extra lifetimes, etc.