Consider rewrite for stable rust

[iankronquist]

Nightly rust is very unstable. The LLVM 6 update broke linking, right now it won't even compile.

I have a file with all the asm stubs & a C header, but we'd still need rust definitions which are slightly different than the current intrinsics.

We'd have to roll our own heap implementation & interface, and that may not work so well. However, we don't use a heap now, so maybe we just won't have one at all.

[iankronquist]

We use the following features which are not available with stable rust:

1. lang_items -- this may change again, but we need a panic implementation.
2. asm -- we may remove this in favor of the x86 crate
3. abi_elfabi -- this will likely get stabilized as is considering its utility and small API surface. It's required by the uefi crate.

[iankronquist]

It looks like the lang items one might actually be stable.
I could get rid of the inline asm, but getting rid of the abi_eflabi unstable feature would be harder.