TS: work in progress:

- SEEK_CUR & SEEK_END i.s.o. plain numbers or home-grown constants
- Left some questions in the code and tests
- _MemIndex refactored (1st attempt; tests say OK - coverage?)
    * key's and blk-indexes are sorted (tested that).
- assert on Array('L').itemsize more meaningful
- Left some pseudo-code for another day to think about:
  nextLineMustBeSentinel flag set from target_key argument in _readNext.
  (Needs a failing test first!).
- Temporary bail-out of long-running tests until refactored into a
  separate performancetest script.
- KeyIndexTest.testMaxSize fails on "maxSize" argument missing.
  Consider an upper limit instead (if size above that: logging, screaming,
  notify monitoring tools or whatnot).