libstd: Change all `~fn()`s to `proc`s in the standard library.

This makes `Cell`s no longer necessary in most cases.
libextra: Remove `~fn()` from libextra.
librustc: Remove the one use of `~fn()`
librustc: Convert `~fn()` to `proc()` everywhere.
libsyntax: Remove `~fn()` from the language
auto merge of #10561 : pcwalton/rust/procify, r=alexcrichton

r? @alexcrichton