Add a pass-by-copy parameter passing convention

This is intended to solve the problem of how to pass arguments to
constructor functions -- you want to move in rvalues, but not have to
explicitly copy stuff that is not an rvalue. The by-copy passing
convention will ensure the callee gets its own copy of the value. For
rvalues, it'll just pass off the value. For lvalues, it'll make a
copy.

Issue #1177

src/comp/util/ppaux.rs

@@ -12,9 +12,10 @@ import middle::ast_map;
 fn mode_str(m: ty::mode) -> str {
     alt m {
       ast::by_ref. { "&&" }
-      ast::by_val. { "+" }
+      ast::by_val. { "++" }
       ast::by_mut_ref. { "&" }
       ast::by_move. { "-" }
+      ast::by_copy. { "+" }
       _ { "" }
     }
 }