Based on what musl does.

wow i am stupid for writing this code

For some reason, C handles `==` before `&`.
So `a & b == c` is the same thing as `a & (b == c)`.

cbindgen can't handle negative numbers, see https://github.com/eqrion/cbindgen/issues/205

Doesn't link yet due to "multiple definitions of malloc", because some are supplied by some library of bash itself. Really odd.

I really really wish I could actually test this on redox. All I know is: it compiles

Transmuting from an immutable to a mutable reference is undefined
behavior in Rust, with the exception of UnsafeCell which tells the
optimizer to not draw too many conclusions. Sadly UnsafeCell::get cannot
yet be used in statics (see https://github.com/rust-lang/rust/issues/51718),
so this works around that by making the statics into functions and
making C macros to call those.

Remove the broken Rust functions and instead provide C macros in a
`include/bits` header. The C macros were taken from musl.