Remove ~, add Box

[alexcrichton]

Tracking issue for rust-lang/rfcs#59

[alexcrichton]

Nominating.

[pnkfelix]

Assigning P-backcompat-lang, 1.0.

[sfackler]

Down with sigils!

[alexcrichton]

There's actually one more thing remaining, ~self, and I believe @pcwalton wanted to keep this open to track the progress on that (but we're almost there!)