Remove RefCell::{with, with_mut}

These are superfluous now that we have fixed rvalue lifetimes and Deref.
Clean up marker types and Unsafe initialization
Copy-edit a sentence about borrowing references
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?
std: Remove the get() method from RefCell wrappers

This method has been entirely obsoleted by autoderef, so there's no reason for
its existence.
syntax: Fix fallout of removing get()
arena: Fix fallout of removing get()
rustc: Fix fallout of removing get()
rustdoc: Fix fallout of removing get()
test: Fix fallout of removing get()
rustc: Remove all usage of manual deref()

Favor using '*' instead
rustuv: Remove usage of get()