Replace libc dependency with core::ffi::*

[cpu]

Presently rustls-ffi takes a dependency on the libc crate for the following imports:

• size_t
• c_void
• c_char
• c_uint
• c_int
• EINVAL
• EIO

It looks like the core::ffi module has many of these same defines, and could potentially let us drop an external dependency.

Blockers:

• core::ffi::c_size_t is unstable (upstream tracking issue).
• EINVAL and EIO aren't present; I think we would need our own platform-gated constants for these.

I propose we wait for c_size_t to stablize and then replace libc with core::ffi::* and manually defined EINVAL/EIO.