This replaces unr with null on disk, though note both the rm bit and unr
are used in-device still, they just don't get written to disk.
This removes the need for the rm bit on disk. Since we no longer need to
figure out what's been removed during fetch, we can save this bit for both
internal and future on-disk use.
Special handling of alta allows us to avoid emitting an unr tag (now null) if
the current trunk is truly unreachable. This is minor now, but important
for a theoretical rbyd rebalance operation (planned), which brings the
rbyd overhead down from ~3x to ~2x.
These changes give us two ways to terminate trunks without a tag:
1. With an alta, if the current trunk is unreachable:
altbgt 0x403 w0 0x7b
altbgt 0x402 w0 0x29
alta w0 0x4
2. With a null, if the current trunk is reachable, either for
code convenience or because emitting an alta is impossible (an empty
rbyd for example):
altbgt 0x403 w0 0x7b
altbgt 0x402 w0 0x29
altbgt 0x401 w0 0x4
null
Christopher Haster
7180b70c9c