Remove RefCell::get

[erickt]

I'm not sure if this deserves a proper RFC or not, but since I consider this to be a bug I figure it makes sense here :)

I just ran into a bug where I assumed that RefCell::get follows our convention of .get() methods returning a &T pointer, and I was surprised to find that RefCell::get is actually:

impl<T:Clone> RefCell<T> {
    pub fn get(&self) -> T {
        (*self.borrow()).clone()
    }
    ...
}

It's surprising, and it can be easily replaced with x.borrow().clone() if a copy is actually needed. I propose we just remove it.

[alexcrichton]

I believe that this was originally implemented to ease the transition from @mut to @RefCell, but with autoderef I don't believe that this is necessary any more. I'd vote for removing it.

[sfackler]

set should go as well.

[nikomatsakis]

👍 but cc #13159