Parentheses notation should respect lifetime elision

[nikomatsakis]

HRTB doesn't correctly implement lifetime elision for Fn() notation. See the test src/test/compile-fail/unboxed-closure-sugar-equiv.rs.

[nikomatsakis]

Nominating.

[pnkfelix]

@alexcrichton notes that this can be fixed backwards compatibly, and @nikomatsakis agrees.

P-high, not 1.0.

[nikomatsakis]

Well, sort of -- I mean lifetime elision broke code, but almost only bad code. The problem is that people can use transmute to satisfy the signature and then clients get to assume any lifetime they want.