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
auto merge of #13053 : alexcrichton/rust/removing-ref-cell-get, r=huonw

This commit removes the `get()` method from `Ref` and `RefMut` in favor of the `*` operator, and removes all usage of the `deref()` function manually from rustc, favoring using `*` instead.

Some of the code is a little wacky, but that's due to either #13044 or #13042