lib: introduce BPS_TREE_IDENTICAL custom comparator

Introduce a macro BPS_TREE_IDENTICAL for BPS TREE class. This
makes possible to define custom comparators for stucture-based
leafs.
Previously, a C++ comparison operator "!=" override was used for
this purpose. Due to the fact that we are not going to rework on
C++ C-only components of Tarantool like memtx_tree, we needed a
way to make complex structures comparisons using preprocessor.

Needed for #3961