Ensure minimal places are captured

[arora-aman]

If we need to capture a.b.c and a.b, then we only capture a.b

With/After #7

[arora-aman]

During upvar analysis, we can keep a map from root variable to paths captured. As we see a new place that needs to be captured we can check if the root variable has a path that has been previously captured, if not just add it.

If the root variable has been captured we can iterate all captured paths and look for parent/child/sibling relations.
eg:

A	B	Relation	What we keep
foo.x.y	foo. x.z	A is sibling of B, since paths diverged	Both
foo.x.y	foo.m	A is sibling of B *, since paths diverged	Both
foo.x	foo. x.z	A is parent of B	A
foo.x.z	foo. x	A is child of B	B
foo.x[20]	foo. x[30]	A == B**	foo.x
foo.x[20].p	foo. x[30].q	A == B**	foo.x
foo.x	foo. x[30]	A == B**	foo.x

* foo.x.y and foo.m are not really siblings, but in our case it just indicates that we need to keep both
** We can't differentiate b/w two different array elements as being disjoint

Initially we can store the information as a mapping from root variable to list of pairs, where each pair is (Place, CaptureInfo).

To optimize answering these questions we can use Tries, but that should be after we stabalize this. It might just add overhead because I don't think we will be capturing crazy long paths, though might be useful if the closure captures a lot of different paths starting at the same root variable.

We also need to be careful about borrow kinds
eg:

let mut rect= Rectangle { top_left: Point { x: 10, y: 20 }, bottom_right: Point { x: 40, y: 50 } };
let c = || { 
      rect.top_left.x += 10;
      println!("{:?}", rect.top_left);
}

We will capture rect.top_left.x as mutable borrow and rect.top_left as immutable borrow. Overall we want to capture rect.top_left as mutable borrow. This would be similar as handling different kinds of use of the same place.

[nikomatsakis]

Status update in #7 (comment)