Add a statically-initializable mutex type to libstd

[sfackler]

There are certain cases where you need a global mutex (see #9713 for one example) and the only reasonable way to do that without something like Go's init functions is a mutex type with a static initializer similar to PTHREAD_MUTEX_INITIALIZER from libpthread.

[alexcrichton]

Closing as a dupe of #9105. One of the requirements listed in that issue is to have statically initialized variants, so I believe that closing that would close this as well.

I think that we're going to have to have a statically initialized mutex which is "slow on the first lock" and "fast everywhere else" which I personally think is OK. Having a statically initialized mutex would be very nice to have though (even as just a proof of concept).