Implement `Fn`, `FnMut`, `FnOnce` traits and make them callable via the `()` operator

[pcwalton]

This is part of unboxed closures. cc @nikomatsakis

Per our discussions last week I believe that this is necessary for 1.0 backcompat-lang (because we will remove the old closures afterward). Nominating.

[brson]

1.0 backcompat lang. Note though that this particular solution has not been accepted as an RFC yet - we just want to solve unboxed closures for 1.0

[pcwalton]

Landed as #14590.

[brson]

\o/