More bounds checking

[blaine]

When building a UCAN, it's really easy (esp. from js vs ts) to incorrectly set fields; e.g.,

ucan.build({ ..., facts: { xyz: "abc" } })

will silently omit the facts, as will

ucan.build({ ..., facts: [ { "xyz": "abc" } ] })

This is probably true for a variety of other fields, too. I would suggest adding explicit checks and refusing to build a UCAN if the fields aren't properly formatted, including errors for why the UCAN creation failed. Happy to take on this work if others are in agreement, but it might be worth waiting until we update to 0.10. Just leaving this issue here in the meantime, nothing urgent.