box: introduce JSON Indexes

New JSON indexes allows to index documents content.
At first, introduced new key_part fields path and path_len
representing JSON path string specified by user. Modified
tuple_format_use_key_part routine constructs corresponding
tuple_fields chain in tuple_format::fields tree to indexed data.
The resulting tree is used for type checking and for alloctating
indexed fields offset slots.

Then refined tuple_init_field_map routine logic parses tuple
msgpack in depth using stack allocated on region and initialize
field map with corresponding tuple_format::field if any.
Finally, to proceed memory allocation for vinyl's secondary key
restored by extracted keys loaded from disc without fields
tree traversal, introduced format::min_tuple_size field - the
size of tuple_format tuple as if all leaf fields are zero.

Example:
To create a new JSON index specify path to document data as a
part of key_part:
parts = {{3, 'str', path = '.FIO.fname', is_nullable = false}}
idx = s:create_index('json_idx', {parts = parse})
idx:select("Ivanov")

Part of #1012