std: rewrite Hash to make it more generic

This patch merges IterBytes and Hash traits, which clears up the
confusion of using `#[deriving(IterBytes)]` to support hashing.
Instead, it now is much easier to use the new `#[deriving(Hash)]`
for making a type hashable with a stream hash.

Furthermore, it supports custom non-stream-based hashers, such as
if a value's hash was cached in a database.

This does not yet replace the old IterBytes-hash with this new
version.

src/etc/generate-deriving-span-tests.py

@@ -119,7 +119,8 @@ for (trait, supers, errs) in [('Rand', [], 1),
                               ('Clone', [], 1), ('DeepClone', ['Clone'], 1),
                               ('Eq', [], 2), ('Ord', [], 8),
                               ('TotalEq', [], 1), ('TotalOrd', ['TotalEq'], 1),
-                              ('Show', [], 1)]:
+                              ('Show', [], 1),
+                              ('Hash', [], 1)]:
     traits[trait] = (ALL, supers, errs)
 
 for (trait, (types, super_traits, error_count)) in traits.items():