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/log_syntax.rs

@@ -18,7 +18,7 @@ use std::rc::Rc;
 pub fn expand_syntax_ext(cx: &mut base::ExtCtxt,
                          sp: codemap::Span,
                          tt: &[ast::TokenTree])
-                      -> ~base::MacResult {
+                         -> Box<base::MacResult> {
 
     cx.print_backtrace();
     println!("{}", print::pprust::tt_to_str(&ast::TTDelim(