Readable delegation manager

[danfinlay]

What?

Adds a new SafeDelegationManger which receives its caveats in a human-readable form, and decodes them into the usual form for enforcement.

Why?

When considering how readable these delegations could be on a 3rd party hardware wallet that only supports ERC-712, we realized the caveat enforcers were falling short of where we'd ideally have their readability.

It seemed like a maximally security conscious user of a hardware wallet might prefer a contract account that ensured they had improved readability on the hardware wallet in exchange for gas cost.

How?

Subclasses the usual DelegationManager, and first transforms its received ReadableDelegation objects into traditional Delegations.

Is only compatible with caveat enforcers that:

• Implement the new _convertReadableTermsToCaveats method.
• Is registered on the SafeDelegationManager's termsToEnforcer map, creating a limited, permissioned set of caveats.

As a bonus, I made it throw an error if it receives an empty caveats array, to minimize another footgun that is lying around.

[danfinlay]

Could have gas performance improved by re-implementing the redeemDelegation method, to avoid decoding twice.

[berardpb]

Missing an "a" in "manager"?