Add option::get_mut_ref() and friends.

[bblum]

With the new borrow-checker, &mut T is noncopyable, so the following functions are now safe:

fn get_mut_ref<'a,T>(x: &'a mut Option<T>) -> &'a mut T { 
    match x { 
        &Some(ref mut inner) => inner,
        &None => fail!(),
    }   
}
fn map_mut<T,U>(x: &mut Option<T>, blk: &fn(&mut T) -> U) -> Option<U> {
    match x { 
        &Some(ref mut inner) => Some(blk(inner)),
        &None => None
    }   
}

[alexcrichton]

I know that get_mut_ref already exists at least, and it seems like map_mut is more powerfully enabled by map_consume?

[thestinger]

It seems we actually already have get_mut_ref. The map_mut method might not be needed since we have mutate and mutate_default for in-place mutation.

[bblum]

Oh cool, I didn't know get_mut_ref existed. map_mut would be needed for times when you don't have ownership of the data, only a mutable reference.

[alexcrichton]

@bblum, but in map_mut you're replacing the data immediately, so why not move out into the blk parameter (which is what map_consume does)

[bblum]

@alexcrichton My thinking is that sometimes you can't use map_consume because all you have to start is an &mut Option<T>.

[alexcrichton]

Oh my I never saw that, disregard me...