Remove nightly dependencies

[ErichDonGubler]

This issue will probably be pretty long-lived, as there are several features we use that rely upon nightly:

• #![feature(const_fn)]
• #![feature(fnbox)]
• #![feature(libc)] <- perhaps we could fix this by using the libc crate?
• #![feature(refcell_replace_swap)] <- waiting for landing in stable
• #![feature(trace_macros)] <- This could probably be removed easily.
• #![feature(alloca)] <- We almost certainly don't actually need this...
• mozjs is currently nightly-only.

[ricochet1k]

trace_macros doesn't need to stay there, it is only used occasionally for macro debugging.
fnbox is used in a few places because it seems impossible to send fns across thread boundaries otherwise.
refcell_replace_swap is probably not necessary.
I don't remember what const_fn is, and I don't know anything about libc.

[ErichDonGubler]

@ricochet1k: const_fn is being used in a few places (looks like mostly jsclass.rs) -- not sure if we need it yet.

[ErichDonGubler]

@ricochet1k: refcell_replace_swap is going to land in stable soon -- it looks like it'll enter beta with the next release: rust-lang/rust#46517

[ErichDonGubler]

Added alloca, since I see that was added.

Seems like we don't really need it, though?

[sagudev]

I think this issue can be closed now. #32