* remove "continuation" special case?
* trie: make offset unsigned and use "-" always. Saves 1 bit. Root
  needs special handling, though. Better leave this alone.
* introduce a mapping char<->int and use only the ints
  internally. Advantages: saves memory when hardcoding <= 32 letters
  (bitset<32>); allows "Qu" tiles or similar.
* fix inconsistencies in the use of "tile" and "letter".
* do the '#' check at a place where it costs less
* Blocks in layout?
* The C++ committe just decided to specify that elements of a vector<>
  are adjacent in memory. So I should check if every STL
  implementation provides this and just use vector<> in WordListTrie.