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/ext/concat.rs

@@ -18,7 +18,8 @@ use std::strbuf::StrBuf;
 
 pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
                          sp: codemap::Span,
-                         tts: &[ast::TokenTree]) -> ~base::MacResult {
+                         tts: &[ast::TokenTree])
+                         -> Box<base::MacResult> {
     let es = match base::get_exprs_from_tts(cx, sp, tts) {
         Some(e) => e,
         None => return base::DummyResult::expr(sp)