Move `std::libc` to standalone `libc`, and rename all `c_type` to `c::type`

[liigo]

I would like to do this work later, if no objection.

[huonw]

cc #12469 for some similar discussion.

However, unfortunately, if we move c_vec and c_str to cffi (or whatever), then either std depends on that (and so cffi can't even use Option or any ~/proc/etc. allocations), or cffi depends on std and so std cannot use anything from cffi (which is essentially infeasible, since std interacts with the OS).

[thestinger]

c_vec and c_str aren't provided by the C standard library so they wouldn't belong there. This should be done with enough foresight to provide backwards compatibility when libclang replaces these hard-wired ABIs at a compiler level.

[emberian]

libc is its own crate now.