* Replace deposit signatures with SignatureBytes, a struct which lazyly parsers signatures only on demand.
* check byte length when parsing SignatureBytes
* add comment to struct
* distinguish BadSignature and BadSignatureBytes in verify_deposit_signature
* add test for valid signature
* Implements TryInto<Signature> for &SignatureBytes and From<Signature> for &SignatureBytes
* add and use PublicKeyBytes + fix formatting
* fix compiler warning + docs for macro generated structs
* adds tests to ensure correct byte lengths
* small style improvement as suggested by michaelsproul
