Use SignatureBytes and PublicKeyBytes for deposits (#472)

* 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

eth2/state_processing/src/per_block_processing/errors.rs

@@ -349,6 +349,8 @@ pub enum DepositInvalid {
     BadIndex { state: u64, deposit: u64 },
     /// The signature (proof-of-possession) does not match the given pubkey.
     BadSignature,
+    /// The signature does not represent a valid BLS signature.
+    BadSignatureBytes,
     /// The specified `branch` and `index` did not form a valid proof that the deposit is included
     /// in the eth1 deposit root.
     BadMerkleProof,