SI-6443 Widen dependent param types in uncurry

Bridge building operates on unusual method signatures:
after uncurry, so parameter lists are collapsed; but before
erasure, so dependently typed parameters are still around.

Original:

    def foo(a: T)(b: a.type, c: a.U): Unit

During computeBridges:

    (a: T, b: a.type, c: a.U)Unit

This signature no longer appears to override the corresponding
one in a superclass, because the types of `b` and `c` are dependent
on method parameters.

The root of the problem is uncurry, which leaves the trees in
a poor state. This commit changes uncurry to remedy this.

An example illustrates it best:

    // source
    def foo(a: A)(b: a.type): b.type = b

    // post uncurry before this patch.
    // not well typed code!
    def foo(a: A, b: a.type): b.type = {

    // post uncurry after this patch
    def foo(a: A, b: A): b.type = {
       val b$1 = b.asInstanceOf[a.type]
       b$1
    }

This PR replaces #1696, and also closes SI-6135

Review by @adriaanm