Enable delegations from contract accounts

[danfinlay]

There are two basic ways:

• Just add an EIP-1271 validator in the signature verification logic loop (if the account has code deployed, call the 1271 validation message)
• EIP-4337 compatibility (Involves much larger changes to Delegatable, as the 4337 relay handles multiple things Delegatable does today, including batching).

Related to: delegatable/delegatable-sol#29