Ensure compatibility with Casper Type V3.0 changes

See: https://docs.google.com/document/d/1M5uux_Ry4HzaD46DL9q1vd-M_QSqpMrcxTjHh3NgfxU/edit

### Casper Types v3.0.0 changes.

#### Added
Add new bytesrepr::Error::NotRepresentable error variant that represents values that are not representable by the serialization format.

Add new Key::Unbond key variant under which the new unbonding information (to support redelegation) is written.

Add new Key::ChainspecRegistry key variant under which the ChainspecRegistry is written.

Add new Key::ChecksumRegistry key variant under which a registry of checksums for a given block is written. There are two checksums in the registry, one for the execution results and the other for the approvals of all deploys in the block.

Add new StoredValue::Unbonding variant to support redelegating.

Add a new type WithdrawPurses which is meant to represent UnbondingPurses as they exist in current live networks.

#### Changed
Extend UnbondingPurse to take a new field new_validator which represents the validator to whom tokens will be re-delegated.

Increase DICTIONARY_ITEM_KEY_MAX_LENGTH to 128.

Change prefix of formatted string representation of ContractPackageHash from "contract-package-wasm" to "contract-package-". Parsing from the old format is still supported.

Apply #[non_exhaustive] to error enums.

Change Debug output of DeployHash to hex-encoded string rather than a list of integers.

#### Fixed
Fix some integer casts, where failure is now detected and reported via new error variant NotRepresentable.