lua-multiret: add multi-return support to a lua stored procedure
https://blueprints.launchpad.net/tarantool/+spec/lua-multiret
lua-multret: review fixes

Don't allow a mix of scalars and complex returns in Lua multireturn.
Allow a mix of tables and tuples in Lua multireturn table.
Merge branch 'lua-multiret'

Conflicts:
	test/box/lua.result