librustc: Remove ~EXPR, ~TYPE, and ~PAT from the language, except

for `~str`/`~[]`.

Note that `~self` still remains, since I forgot to add support for
`Box<self>` before the snapshot.

How to update your code:

* Instead of `~EXPR`, you should write `box EXPR`.

* Instead of `~TYPE`, you should write `Box<Type>`.

* Instead of `~PATTERN`, you should write `box PATTERN`.

[breaking-change]

src/libsyntax/ast_util.rs

@@ -121,7 +121,7 @@ pub fn is_shift_binop(b: BinOp) -> bool {
 pub fn unop_to_str(op: UnOp) -> &'static str {
     match op {
       UnBox => "@",
-      UnUniq => "~",
+      UnUniq => "box() ",
       UnDeref => "*",
       UnNot => "!",
       UnNeg => "-",