This patch creates tuple_formats for the tuples obtained through
the netbox.

Closes #2978

@TarantoolBot document
Title: Field names for tuples received from net.box

It is possible now to access by field name for tuples received
from net.box. For example:

box.cfg{listen = 3302}
box.schema.user.grant('guest','read, write, execute', 'space')
box.schema.user.grant('guest', 'create', 'space')

box.schema.create_space("named", {format = {{name = "id"}}})
box.space.named:create_index('id', {parts = {{1, 'unsigned'}}})
box.space.named:insert({1})
require('net.box').connect('localhost', 3302).space.named:get(1).id

Result:

tarantool> require('net.box').connect('localhost', 3302).space.named:get(1).id
---
- 1
...