std::vec: remove unnecessary count parameter on {bytes,
raw}::copy_memory.

Slices carry their length with them, so we can just use that
information.
std::vec::bytes: remove obsolete functions.

These are less useful versions of the comparison operators and TotalOrd
trait.
std::vec::raw: clean up docs.
std::vec: move pointless `raw::get` and `unsafe_get` functions.

This can easily be written as `(*v.unsafe_ref(i)).clone()`, or just
`*v.unsafe_ref(i)` for primitive types like `i32` (the common case).
Move std::{str,vec}::raw::set_len to an unsafe method on Owned{Vector,Str}.
std::vec: convert to(_mut)_ptr to as_... methods on &[] and &mut [].
auto merge of #10984 : huonw/rust/clean-raw, r=cmr

See commits for details.