minor, provide backend that maps our timeline into a file on disk
minor, materialise rb.h red-black trees so they coincide with the disk layout
minor, support dsk backend in bitte API
test, provide simple add/get regression for dsk backend
hygiene, operate in terms of base ptr and offsets for rb-trees in bitte-dsk
hygiene, use ftnd_t instead of uint32_t
hygiene, rethink the responsibilities of rb_* and ftmap_* code

Let ftmap_* routines do the allocation stuff, because rb_* routines
should not really know about it, other than it's base + ftnd_t to
derive a struct ftnd_s*.

On the other hand, the struct ftnd_s* should be found nowhere outside
of rb_* routines.
minor, align a page's fht with base of ftmap
hygiene, call the ftmap finaliser from _close() routine
fix, PEBKAC, when O_TRUNC is requested make sure we truncate
Merge branch 'backend/dsk'

* backend/dsk:
  fix, PEBKAC, when O_TRUNC is requested make sure we truncate
  hygiene, call the ftmap finaliser from _close() routine
  minor, align a page's fht with base of ftmap
  hygiene, rethink the responsibilities of rb_* and ftmap_* code
  hygiene, use ftnd_t instead of uint32_t
  hygiene, operate in terms of base ptr and offsets for rb-trees in bitte-dsk
  test, provide simple add/get regression for dsk backend
  minor, support dsk backend in bitte API
  minor, materialise rb.h red-black trees so they coincide with the disk layout
  minor, provide backend that maps our timeline into a file on disk
fix, PEBKAC, as an estimate for ntrans use the number of materialised pages
minor, extend the disk file after hitting NXPP transactions
test, provide pumping tool for dsk backend
minor, incorporate rotation into rb_insert(), it's not used outside of it

And also helps clarifying our statement about the restricted base ptr.
minor, secure against rotating the NIL pointer
minor, actually free the store upon _close()
hygiene, be consistent about offset signs
minor, initialise a node by memset()'ing the space to -1
Revert "minor, secure against rotating the NIL pointer"

This reverts commit 072e851f16aae1ec607caebe2030a391e5f70a35.