move into musl dir

[burrbull]

Preparing for adding alternatives like newlib.

[alexcrichton]

I'm a bit out of the loop wrt newlib/musl, mind giving me a bit of a brief overview of the intended course of action here? I'm not sure what the purpose of the musl feature is in that I feel like we'd probably want to always pick some implementation to give to users, and we could weigh newlib vs musl if we'd like?

[burrbull]

#138

[alexcrichton]

FWIW I don't have a ton of time to help maintain this library, but I want to make sure it's not entirely forgotten since platforms rely on it. I'd personally appreciate answers to questions which are a bit more expansive if possible than a curt "here's an issue number" link.]

If the goal here is simply speed, why is a new directory of musl implementations needed? We're always only going to have one implementation of each function, and it could be annotated as to where it came from presumably?

[mati865]

cc @japaric