check proofs when build new ucan

[hugomrdias]

I wonder if it's worth putting a check here to ensure that the proofs are encoded? Or even automated encoding of them if they're not, such as:

proofs = proofs.map((prf) => isUcan(prf) ? prf : serialize(prf));

It feels like it would be good to catch that kind of rookie error at this point, rather than building the UCAN and then the user getting an error thrown when they later try to use it.

Originally posted by @adamalton in #3 (comment)