std: Make atomics immutable. #11583

In Rust, the strongest guarantee that `&mut` provides is that the memory
pointed to is *not aliased*, whereas `&`'s guarantees are much weaker:
that the value can be aliased, and may be mutated under proper precautions
(interior mutability).

Our atomics though use `&mut` for mutation even while creating multiple
aliases, so this changes them to use 'interior mutability', mutating
through immutable references.
std: Make the generic atomics take unsafe pointers

These mutate values behind references that are Freeze, which is not
allowed.
std: Make the generic atomics in `sync::atomics` private

I'm not comfortable exposing public functions that purport to do
atomic operations on arbitrary T.
std: Remove AtomicU64

Support for this is less universal than for word-size things;
it has no users; i'd rather play it safe.
std: Update atomic documentation to remove 'mut'

It's all no longer necessary
std: Rename {push,read}_bytes to {push,read}_exact

These methods can be mistaken for general "read some bytes" utilities when
they're actually only meant for reading an exact number of bytes. By renaming
them it's much clearer about what they're doing without having to read the
documentation.

Closes #12892
Remove RefCell::{with, with_mut}

These are superfluous now that we have fixed rvalue lifetimes and Deref.
std: Implement Clone/TotalEq for ProcessExit

It's useful for structures which use deriving(Clone, TotalEq), even though it's
implicitly copyable.

Closes #13047
Clean up marker types and Unsafe initialization
Copy-edit a sentence about borrowing references
auto merge of #13036 : alexcrichton/rust/atomics, r=alexcrichton

Closes #11583, rebasing of #12430 now that we've got `Share` and better analysis with statics.
auto merge of #13051 : alexcrichton/rust/issue-13047, r=thestinger

It's useful for structures which use deriving(Clone), even though it's
implicitly copyable.

Closes #13047
auto merge of #12907 : alexcrichton/rust/issue-12892, r=brson

These methods can be mistaken for general "read some bytes" utilities when
they're actually only meant for reading an exact number of bytes. By renaming
them it's much clearer about what they're doing without having to read the
documentation.

Closes #12892
auto merge of #13052 : sfackler/rust/clean-refcell, r=alexcrichton

These are superfluous now that we have fixed rvalue lifetimes and Deref.

I'd also like to kill off `get` and `set`, but that'll be a large change so I want to make sure that we actually want to do that first.
auto merge of #13062 : mbrubeck/rust/doc-edit, r=alexcrichton

This is a very minor edit to the tutorial section on references.

Reading this section for the first time, I stumbled on the phrase "a reference can be borrowed to any object."  Its meaning was clear enough once I got it, but I had to re-read it a couple of times to parse it correctly.  Something about the passive voice plus the way "reference to any object" is split up by the verb phrase.  How about this instead?
rustc: Remove special treatment for Freeze and NoFreeze

Fixes #12577
arena: Remove Freeze / NoFreeze
sync: Remove Freeze / NoFreeze
std::comm: Remove Freeze / NoFreeze
cell: Remove Freeze / NoFreeze