add validator to Signature struct

[poiresel]

Description

Quick description:

• New features

#228

Changes

Core change: addition of validator to Types.sol

  struct Signature {
    address signatory;            // Address of the wallet used to sign
    address validator;            // Address of the intended swap contract
    bytes1 version;               // EIP-191 signature version
    uint8 v;                      // `v` value of an ECDSA signature
    bytes32 r;                    // `r` value of an ECDSA signature
    bytes32 s;                    // `s` value of an ECDSA signature
  }

Smaller changes

Upgrade package.json versions that weren't initially upgraded
Remove NULL_ADDRESS constant usage as it does not exist

Tests

##Test Coverage

[ejwessel]

Overall looks good. Defaulting to Alice for final approval.

[dmosites]

LGTM