json: implement json_tree class

New JSON tree class would store JSON paths for tuple fields
for registered non-plain indexes. It is a hierarchical data
structure that organize JSON nodes produced by parser.
Class provides API to lookup node by path and iterate over the
tree.

JSON Indexes patch require such functionality to make lookup
for tuple_fields by path, make initialization of field map and
build vynyl_stmt msgpack for secondary index via JSON tree
iteration.

@locker:
 - Forbid root = NULL in json_tree_lookup_path.
 - Make loop variable first in foreach macros argument list.
 - Use int instead of uint32_t unless uint32_t is specifically
   required.
 - Set max_child_idx to -1 if children array is empty to avoid
   ambiguity.
 - Fix child index allocation for JSON_TOKEN_STR.
 - Rework json_tree_add and json_tree_del to make them more
   straightforward.
 - Add comments and cleanup code.

Needed for #1012