Reduce usage of libc

[notgull]

I am attempting to reduce the dependence on libc in my indirect
dependencies. While looking at this crate I noticed some low-hanging
fruit that can be very easily replaced with libc.

• The rand() function can be re-implemented as a simple Wyrand seeded by
  the thread ID and the current time.
• libc::memcpy is used at a point where it can be replaced with
  ptr::copy_nonoverlapping.
• libc is used for memory allocation at a point where std::alloc can be
  used instead. So I use this.

I'm not too familiar with this codebase, so if any of these replacements
are incorrect, please let me know.

The only leftover libc call is mprotect().