Reuse monomorphized functions for different box types

The free glue for opaque boxes will pick the actual tydesc out of the
box, and call its glue.

Issue #1736

src/comp/metadata/tydecode.rs

@@ -301,6 +301,7 @@ fn parse_ty(st: @pstate, conv: conv_did) -> ty::t {
         let inner = parse_ty(st, conv);
         ty::mk_named(st.tcx, inner, name)
       }
+      'B' { ty::mk_opaque_box(st.tcx) }
       c { #error("unexpected char in type string: %c", c); fail;}
     }
 }

src/comp/metadata/tyencode.rs

@@ -192,6 +192,7 @@ fn enc_sty(w: io::writer, cx: @ctxt, st: ty::sty) {
         for tc: @ty::type_constr in cs { enc_ty_constr(w, cx, tc); }
         w.write_char(']');
       }
+      ty::ty_opaque_box { w.write_char('B'); }
     }
 }
 fn enc_proto(w: io::writer, proto: proto) {

src/comp/middle/shape.rs

@@ -381,6 +381,10 @@ fn shape_of(ccx: @crate_ctxt, t: ty::t, ty_param_map: [uint]) -> [u8] {
         s += [shape_box];
         add_substr(s, shape_of(ccx, mt.ty, ty_param_map));
       }
+      ty::ty_opaque_box {
+        s += [shape_box];
+        add_substr(s, [shape_u8]);
+      }
       ty::ty_uniq(mt) {
         s += [shape_uniq];
         add_substr(s, shape_of(ccx, mt.ty, ty_param_map));