Implement almost all of printf

See merge request redox-os/relibc!171

wow i am stupid for writing this code

Huge thanks to @xTibor for both discovering that this was still an issue, and also providing information with how to fix it

Add tests for constructors and destructors

See merge request redox-os/relibc!169

Move regoff_t to regex.h

See merge request redox-os/relibc!168

Optimise `memcmp` for speed

See merge request redox-os/relibc!167

As per the comments from jD91mZM2 on the merge request.

Make assert more hygienic

See merge request redox-os/relibc!166

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

Accidentally made file references not count as references and therefore also close the fd. My bad.

I saw that in other parts of the `string` module iterations over `usize` were
used to increase iteration speed.  In this patch I apply the same logic to
`memcmp`.  With this change I measured a 7x speedup for `memcmp` on a ~1MB
buffer (comparing two buffers with the same content) on my machine (i7-7500U),
but I did not do any real world benchmarking for the change.  The increase in
speed comes with the tradeoff of both increased complexity and larger generated
assembly code for the function.

I tested the correctness of the implementation by generating two randomly filled
buffers and comparing the `memcmp` result of the old implementation against this
new one.

I ran the tests and currently currently three of them fail:
  - netdb (fails to run)
  - stdio/rename (fails to verify)
  - unistd/pipe (fails to verify)

They do so though regardless of this change, so I don't think they are related.

Add termios baud rate functions

See merge request redox-os/relibc!165

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

Bug discovered by @xTibor. Test and input data provided by him.

Rewrite IO to use core-io library

See merge request redox-os/relibc!164