Optimise skip-index performance.

[hamishmorgan]

Space could be further reduced using skip indexing, by optimising the order in which features and entries are indexed. This would require additional re-indexing stages to be introduced:

• Firstly after counting. The feature and entry indices should be occur in descending order of frequency. Currently they are loosely ordered by frequency, since high frequency words are very likely to occur soon, but it could be optimised.
• Secondly after filtering, moving all filtered features to high indexes so they never cause gaps in the order indices.

This feature would require substantial reworking, so is probably not worth while unless space usage is considered critical. Space saving is unlikely to excede 25%.