Use the libc crate to provide external functions and c-like types

[software-opal]

I'm using this project to port a large C project and it is amazing.

I am however running into a few issues:

• The use of u8 causes lots of cross compilation problems.
• I would like to avoid large extern blocks at the top of each ported file.

To avoid the former I'd like to switch to using libc::c_char.

For the latter it would be nice to (via a flag, maybe) convert calls to C standard library functions like malloc and free into calls to their libc:: equivalents.