Avoid many CrateConfig clones.

This commit changes `ExtCtx::cfg()` so it returns a `CrateConfig`
reference instead of a clone. As a result, it also changes all of the
`cfg()` callsites to explicitly clone... except one, because the commit
also changes `macro_parser::parse()` to take `&CrateConfig`. This is
good, because that function can be hot, and `CrateConfig` is expensive
to clone.

This change almost halves the number of heap allocations done by rustc
for `html5ever` in rustc-benchmarks suite, which makes compilation 1.20x
faster.

src/libsyntax/ext/source_util.rs

@@ -94,7 +94,7 @@ pub fn expand_include<'cx>(cx: &'cx mut ExtCtxt, sp: Span, tts: &[tokenstream::T
     // The file will be added to the code map by the parser
     let p =
         parse::new_sub_parser_from_file(cx.parse_sess(),
-                                        cx.cfg(),
+                                        cx.cfg().clone(),
                                         &res_rel_file(cx,
                                                       sp,
                                                       Path::new(&file)),