Implement almost all of printf

See merge request !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 !169

Move regoff_t to regex.h

See merge request !168

Optimise `memcmp` for speed

See merge request !167

As per the comments from jD91mZM2 on the merge request.

Make assert more hygienic

See merge request !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 !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 !164