[ot][spam][crazy] crazylog: append-only random-access data

[Undiscussed Horrific Abuse, One Victim of Many]

so a write has two ends, a start and an end, and each one can either
be within an old write, within an old flush, or in a new area.

then similarly, they can span all or parts of writes and flushes, as well.

the old code uses start_idx and end_idx and then excises that range.
the ends are treated specially. the most important part seems to be
adjusting their bounds.
if a write or a flush is partially cut, its bounds can be adjusted,
and if it is a flush then it can be further optimised as it will have
lost leaves.