Optionally make jemallocator register as a #[global_allocator]

[SimonSapin]

Following up on https://internals.rust-lang.org/t/jemalloc-was-just-removed-from-the-standard-library/8759/10, it makes sense to me to add two Cargo features to this crate:

• global_allocator_on_tested_platforms: define the #[global_allocator] with a cfg that only applies to targets this repository has CI for.
• global_allocator: same, but unconditional. It might work on an untested platform.

CC @gnzlbg @sfackler @alexcrichton

[SimonSapin]

Hmm, I just realized that for the conditional one we probably don’t want to link to jemalloc-sys on untested platforms, so it might need to be a separate crate rather than a feature of jemallocator.

[gnzlbg]

I think that we can put the global_allocator (unconditional) feature in jemallocator, but I think it might be worth it to do the "global_allocator_on_supported_platforms on a different crate, so that it only includes jemallocator (and therefore jemalloc-sys) in those platforms in which it is guaranteed to work, and the crate is "empty" otherwise. Bolting this logic onto jemallocator might get messy.

It makes sense to have this other crate in this repo, so that we can reuse all of the CI that's already set up.