(https://blueprints.launchpad.net/tarantool/+spec/compile-with-external-luajit)

Clarify comment in index_count() function.

- Add space_n() and index_n() utility functions.
- Add index_is_primary() utility function.
- Rework index initialization and iteration sequences.
- Coding style fixes here and there.

Conflicts:
	test/CMakeLists.txt

Code review fixes for branch tree-index-opt:
to avoid unnecessary copying and settle the issue of
ownership of key_def structures, move their ownership
to struct space.

Now Index uses its key_def by pointer, and all key_defs are
allocated and freed separately in struct space
allocation/deallocation functions.

Fix coding style (use Lisp/Emacs style function
call style, as per LKML which we follow).

validation added.

- use dedicated comparison functions for each Tree type to avoid method
  dispatch within the functions;
- a number of cosmetic changes.

Valgrind tests was fixed.