Scoping bug with patterns in binders

[mtzguido]

type box t = | Box of t

let unbox (Box x) = x

let works (Box a : box int) (Box x : box int {x > 0}) : pos = x
let fails (Box a : box int) (Box x : box int {a + x > 0}) : pos = a+x

The last definition fails pointing to the a in the refinement with:

* Error 230 at Parsing.fst(8,46-8,47):
  - Variable "a" not found

This also fails the same way:

let fails (Box a : box int) (_ : unit {a > 0}) : pos = a

And this one fails more interestingly:

let fails (Box a : box int) (_ : squash (a > 0)) : pos = a

* Error 189 at Parsing.fst(8,41-8,42):
  - Expected expression of type Prims.int got expression a of type box Prims.int
  - See also Parsing.fst(8,11-8,16)