box: zap key_part_def struct

The only difference between struct key_part_def and struct key_part is
that the former stores only the id of a collation while the latter also
stores a pointer to speed up tuple comparisons. It isn't worth keeping a
separate struct just because of that. Let's use struct key_part
everywhere and assume that key_part->coll is NULL if the part is needed
solely for storing a decoded key part definition and isn't NULL if it is
used for tuple comparisons (i.e. is attached to a key_def).