codegen: remove dead module branchify.
codegen: remove unneeded imports of FromPrimitive and ToPrimitive.
codegen: make the Hash impl's generic over the state type.
codegen: cast enums to integers instead of matching.
codegen: replace .code() calls with direct casts.
codegen: use #[deriving(FromPrimitive)].
Replace codegen with static code, now that it is DRY.
Merge pull request #183 from eddyb/no-codegen

Replace codegen with DRY static code.