Remove domain seperators on KeyHash/ValueHash

[hdevalence]

It's cleaner and better to have domain separators for key and value hashes, so that we can't risk confusing key and value hashes, or a hash from some other part of some other protocol. But unfortunately, making domain separators work with ICS23 requires making changes to the ICS23 spec, and that will take time to roll out and cause incompatibility until the changes are adopted by every other deployment.

Instead, it seems better to take the slight cleanliness/design hit and drop the domain separators, so we don't have any compatibility concerns. The key and value hashes should just be straight SHA256 of their byte values.