document: tell whether a document holds a change without building it

With the policy's reads kept, judging a change still costs about one
read (some 3.5 µs), and so does judging without a policy: what is left
is At::new, which asks get_change_by_hash for each head. Automerge
builds the change again from the document's operations to answer, at
a cost growing with its history. Screening asks it again for every
candidate and every dependency, and sync for heads.

Ask by the missing dependencies instead (changes::holds), which look
the hash up: one the document holds is not among them. A change queued
in the document, waiting for its dependencies, isn't either; but then
some are missing, and only then is it asked the slow way. Every check
of whether a document holds a change goes through it.

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