memtx: introduce tuple compare hint

Implement functions for retrieving tuple hints for a particular
key_def. Hint is an integer that can be used for tuple comparison
optimization: if a hint of one tuple is less than a hint of another
then the first tuple is definitely less than the second; only if
hints are equal tuple_compare must be called for getting comparison
result. Hints are calculated using only the first part of key_def.

@locker:
 - Rework key_def_set_hint_func.
 - Refactor functions calculating hints.
 - Drop has_collation template argument (we don't use templates
   for collations anywhere else).
 - Add part_count argument to key_hint (it's conventional to pass
   part_count along with decoded key).
 - Improve comments, rename a few functions, and cleanup code.

Close #3961