- Add SignerPod.id computation (integrate kvs with MerkleTree).
- Updates frontend::SignerPod to be a wrapper on top of
backend::SignerPod with extra metadata (a keymap between hashes and
their strings).
- Get's rid of SignerPod.compile() since now the frontend::SignerPod
uses the method `::new()` which internally calls
backend::SignerPod::new which constructs the merkletree to use it's root
as PodID.
