u3: moves home road to loom start, simplifies snapshot system#826
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This PR is based on and targets #812. It also includes a copy of #825, as that fix is necessary for the relocated home road to not interfere with inner-road promotion. It implements "option 2" from #451 (comment).
Moving the home road makes sense given #812, as it requires that the entire first page be reserved from the heap. Once the road home is moved, there's no reason for the snapshot to consist of two separage memory-image segments, as only the bottom of the loom needs to be preserved. This simplifies the snapshot system by removing much of the complicated, direction-switching redundancy in its implementation, and simplifies the handling of snapshots themselves as they are now a single file.
Since these changes require a snapshot-system migration, I've taken the opportunity to add and verify a top-level checksum for all snapshot patch metadata.