Feature 'add-delete-fields': add/delete fields with UPDATE

Implement blueprint 'add-delete-fields':
https://blueprints.launchpad.net/tarantool/+spec/add-and-delete-fields-in-tuple

Make it possible to add new fields with UPDATE, and
delete existing fields.

Rewrite the UPDATE algorithm to pre-compute new tuple
length and do most of the work in place, without
allocating temporary buffer for fields.

Add Connector/C support for new UPDATE options.

Add and update tests.

Joint effort by Konstantin Shulgin (initial implementation),
Dmitry Simonenko (Connector/C) and
Konstantin Osipov (spec/code review).