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