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_map.rs

@@ -110,7 +110,7 @@ fn extend(cx: ctx, +elt: ident) -> @path {
 }
 
 fn mk_ast_map_visitor() -> vt {
-    return visit::mk_vt(@{
+    return visit::mk_vt(@visit::Visitor {
         visit_item: map_item,
         visit_expr: map_expr,
         visit_stmt: map_stmt,