lots of work to make iface/impls parameterized by regions

- paths can now take region parameters, replacing the dirty hack
  I was doing before of abusing vstores.  vstores are now a bit
  of a hack though.

- fix various small bugs:
  - we never checked that iface types were compatible when casting
    to an iface with `as`
  - we allowed nonsense like int<int>
  - and more! (actually that may be it)

src/rustdoc/extract.rs

@@ -88,12 +88,12 @@ fn moddoc_from_mod(
                     resdoc_from_resource(itemdoc)
                 ))
               }
-              ast::item_iface(_, methods) {
+              ast::item_iface(_, _, methods) {
                 some(doc::ifacetag(
                     ifacedoc_from_iface(itemdoc, methods)
                 ))
               }
-              ast::item_impl(_, _, _, methods) {
+              ast::item_impl(_, _, _, _, methods) {
                 some(doc::impltag(
                     impldoc_from_impl(itemdoc, methods)
                 ))