This is easy to satisfy given that the template-to-script code takes a
slice of keys. Just do &keys[..n_keys] if you have too many keys. (If
you have too few you're SOL no matter what.) This way we can catch
likely configuration errors without putting much of a burden on users
who legitimately have more keys than the template requires.
Also add a method required_keys() to Template so that users can check
how many keys they ought to have.
Does not do stuff like validating the form of contracts, since this seems like
more of an application thing. Does not even distinguish a "nonce", just assumes
the contract has whatever uniqueness is needed baked in.
Andrew Poelstra