Optimise universe proof leaf additional inputs

[ffranr]

As things currently stand, a universe's proof leaf has a field called AdditionalInputs. This field contains proofs for other inputs that may have been merged as part of a spend.

The AdditionalInputs field is of type []proof.File. Which means, that for each additional input, we store an entire proof file. A universe leaf is supposed to represent a single proof. But where additional inputs are concerned, a universe leaf may also contain entire proof files for each additional input.

This means that a universe makes sub-optimal use of memory in that each leaf may also contain entire proof files which are duplicated as leaves in other parts of the universe.

Within a universe leaf, the AdditionalInputs field should perhaps contain proof locators instead of entire proof files.