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/test/compile-fail-fulldeps/auxiliary/macro_crate_test.rs

@@ -57,7 +57,7 @@ fn expand_identity(cx: &mut ExtCtxt, _span: Span, tts: &[TokenTree])
                    -> Box<MacResult+'static> {
     // Parse an expression and emit it unchanged.
     let mut parser = parse::new_parser_from_tts(cx.parse_sess(),
-        cx.cfg(), tts.to_vec());
+        cx.cfg().clone(), tts.to_vec());
     let expr = parser.parse_expr().unwrap();
     MacEager::expr(quote_expr!(&mut *cx, $expr))
 }

src/test/run-pass-fulldeps/auxiliary/macro_crate_test.rs

@@ -60,7 +60,7 @@ fn expand_make_a_1(cx: &mut ExtCtxt, sp: Span, tts: &[TokenTree]) -> Box<MacResu
 // See Issue #15750
 fn expand_identity(cx: &mut ExtCtxt, _span: Span, tts: &[TokenTree]) -> Box<MacResult + 'static> {
     // Parse an expression and emit it unchanged.
-    let mut parser = parse::new_parser_from_tts(cx.parse_sess(), cx.cfg(), tts.to_vec());
+    let mut parser = parse::new_parser_from_tts(cx.parse_sess(), cx.cfg().clone(), tts.to_vec());
     let expr = parser.parse_expr().unwrap();
     MacEager::expr(quote_expr!(&mut *cx, $expr))
 }