This commit begins the process of integrating `zcash_script` with the rest of the system for verifying scripts while syncing the block chain. It does so by adding the necessary support for looking up UTXOs from the state service and implements the first parts of the `script::Verifier` for looking up the necessary UTXOs in the state and then generating the necessary call to `zcash_script` to verify the script itself.
Co-authored-by: teor <teor@riseup.net>
* Flatten consensus::verify::* to consensus::*
* Move consensus::*::tests into their own files
* Move CheckpointList into its own file
* Move Progress and Target into a types module
QueuedBlock and QueuedBlockList can stay in checkpoint.rs, because
they are tightly coupled to CheckpointVerifier.