Changed most vec! invocations to use square braces

Most of the Rust community agrees that the vec! macro is clearer when
called using square brackets [] instead of regular brackets (). Most of
these ocurrences are from before macros allowed using different types of
brackets.

There is one left unchanged in a pretty-print test, as the pretty
printer still wants it to have regular brackets.

src/libsyntax/print/pprust.rs

@@ -2277,7 +2277,7 @@ impl<'a> State<'a> {
                     Ok(())
                 }));
 
-                let mut options = vec!();
+                let mut options = vec![];
                 if a.volatile {
                     options.push("volatile");
                 }