[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/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)
+    };
+
+}