Generate *.h from libc crate

[lawkai-vivo]

librs is purely implemented in Rust, so it doesn't naturally offer *.h by default. To conform to POSIX standard, we have to mark which header file a type belongs to in the libc crate. Thus we can use this information to export c headers from the libc crate.

[lawkai-vivo]

Roughly we need a proc macro to get the work done. Like

#[export_to = "stdio.h"]
#[repr(C)]
pub struct bar { ... }