Meta: Borrow checker rules rewrite

[nikomatsakis]

The current borrow checker rules are not quite sound and also insufficient expressive. They don't support nested calls nor are they flow-sensitive. I have a rewrite in progress to address these 3 issues.

[thestinger]

Here's a test case for the issue with multiple ref mut patterns, that I ran into in writing find_mut for the trie:

enum Foo {
    A(int), B(int), C
}

fn main() {
    let mut x = A(5);
    let res = match x {
        A(ref mut a) => Some(a),
        B(ref mut b) => Some(b),
        C => None
    };
}

[nikomatsakis]

I am going to treat this as a meta-issue, since it is evident that not everything will be landed in one go.

One sub-issue is #4903 (Flow sensitivity)

[nikomatsakis]

A related issue is #4384 and #5016, since I think that move checking should move into the borrow checker

[nikomatsakis]

A related issue is #3387, which concerns moves in overloaded operators (this should be fixed on my branch).

[nikomatsakis]

A related issue is #3850, also addressed in my branch.