A collection of refactorings that I found it hard/tiresome to divide:

- Make `extern fn()` assignable to any closure type, rather than
  a subtype.
- Remove unused int_ty_set and float_ty_set
- Refactor variable unification and make it more DRY
- Do fn sub/lub/glb on the level of fn_sig
- Rename infer::to_str::ToStr to infer::to_str::InferStr
- Capitalize names of various types
- Correct hashing of FnMeta
- Convert various records-of-fns into structs-of-fns.  This is both
  eliminating use of deprecated features and more forwards compatible
  with fn reform.

r=pcwalton

src/libsyntax/ast_util.rs

@@ -440,10 +440,8 @@ fn empty(range: id_range) -> bool {
 }
 
 fn id_visitor(vfn: fn@(node_id)) -> visit::vt<()> {
-    visit::mk_simple_visitor(@{
-        visit_mod: fn@(_m: _mod, _sp: span, id: node_id) {
-            vfn(id)
-        },
+    visit::mk_simple_visitor(@visit::SimpleVisitor {
+        visit_mod: |_m, _sp, id| vfn(id),
 
         visit_view_item: fn@(vi: @view_item) {
             match vi.node {