treesearch improvements

- treesearch:
  - remove support for mapping indices of dact corpora, in favor
    of renumbering and padding sentence IDs beforehand
  - macro support for regex queries
- treetransforms: option to renumber the sentence IDs in a corpus,
  padded with 8 spaces.
- web/treesearch.py:
  - expect trees in dact corpora to be renumbered
  - tokenize with ucto if possible, to retain paragraph breaks
  - use readability module instead of style(1) command
  - base readability measures on equal number of sentences for each text
  - sort results for sentences / trees by corpus name
- treebank: return alpino / dact blocks as bytes strings