Don't hash contract storage keys after encrypting

[armaniferrante]

Instead we can just slice the first 32 bytes.

Even better, we can put the encrypted key directly into storage as our storage supports it. However, this would require rewriting all parity state/accounts to support arbitrary length keys, instead of H256.

See here for current implementation.

Note: this requires a state wipe. We should do this in the same pass that we swap out mrae.

For more background, see https://oasis-labs.slack.com/archives/GAX99F0A0/p1553309277351300

[bennetyee]

See https://github.com/oasislabs/private-rfcs/pull/100 -- do not use mrae.seal for keys.