Shorten RefCell's borrow() and borrow_mut()


A small change to the names of these functions to improve ergonomics.

borrow() -> get() borrow_mut() -> get_mut()


The intent of these names is to advertise they work like other borrows. In particular, it’s not obvious that you can’t execute another get_mut after you have already done a get_mut, and the type system won’t help you…