[libstd] getopts, now with fewer copies

Change the opt_ functions in getopts to take a reference to a
Matches, instead of taking a Matches by-value, as suggested in

src/test/bench/shootout-pfib.rs

@@ -53,7 +53,7 @@ fn parse_opts(argv: ~[~str]) -> config {
     let opt_args = vec::slice(argv, 1u, vec::len(argv));
 
     match getopts::getopts(opt_args, opts) {
-      Ok(m) => { return {stress: getopts::opt_present(m, ~"stress")} }
+      Ok(ref m) => { return {stress: getopts::opt_present(m, ~"stress")} }
       Err(_) => { fail; }
     }
 }

src/test/run-pass/getopts_ref.rs

@@ -0,0 +1,15 @@
+extern mod std;
+
+use std::getopts::*;
+
+fn main() {
+    let args = ~[];
+    let opts = ~[optopt(~"b")];
+
+    match getopts(args, opts) {
+        result::Ok(ref m)  =>
+            assert !opt_present(m, "b"),
+        result::Err(f) => fail fail_str(f)
+    };
+
+}