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/libcore/stackwalk.rs

@@ -99,5 +99,5 @@ extern mod rustrt {
 #[abi = "rust-intrinsic"]
 extern mod rusti {
     #[legacy_exports];
-    fn frame_address(f: fn(++x: *u8));
+    fn frame_address(f: &once fn(++x: *u8));
 }