`SpaceTransaction` should take `ReadTicket` on check instead of commit

Whenever a `SpaceTransaction` inserts a block, it needs a `ReadTicket` to evaluate those blocks. It is generally awkward to provide such a `ReadTicket` since it is for “everything but this `Space`”. Additionally, we might under some circumstances prefer to make the transaction check fail if block evaluation fails. Therefore, `SpaceTransaction` (and the transaction traits) should be modified so that the `ReadTicket` may be supplied at `check()` time, and the check performs the evaluations and passes their results to the commit. Hooking up change notifications will be tricky.