Optimise the datacap actor's allowance data into the same HAMT as balances

[anorth]

This is a tracking issue for filecoin-project/actors-utils#188.

Instrumentation suggests that changing the state schema for the datacap actor could lead to a significant reduction in gas used for the token transfers associated with verified deals.

[anorth]

This is a fairly large change that I don't think fits well with a fast-follow nv19. I'm removing from that project.

[alexytsu]

Now that the datacap calls have themselves been batched as a part of #1278 there is only one call per PSD. The call to datacap accounts for around 1% of the gas of the entire routine so any internal optimisations here are unlikely to move the needle in the short term.

[anorth]

Thanks, I'm going to close this as wontfix. This is now only a small part of any built-in routines that use datacap. It could still be beneficial for new uses of the token library to combine this state, but not worth migrating datacap at the moment.