box: specify indexes in user-friendly form

Implemented a more convenient interface for creating an index
by JSON path. Instead of specifying fieldno and relative path
it is now possible to pass full JSON path to data.

Closes #1012

@TarantoolBot document
Title: Indexes by JSON path
Sometimes field data could have complex document structure.
When this structure is consistent across whole space,
you are able to create an index by JSON path.

Example:
s = box.schema.space.create('sample')
format = {{'id', 'unsigned'}, {'data', 'map'}}
s:format(format)
-- explicit JSON index creation
age_idx = s:create_index('age', {{2, 'number', path = "age"}})
-- user-friendly syntax for JSON index creation
parts = {{'data.FIO["fname"]', 'str'}, {'data.FIO["sname"]', 'str'},
     {'data.age', 'number'}}
info_idx = s:create_index('info', {parts = parts}})
s:insert({1, {FIO={fname="James", sname="Bond"}, age=35}})