api: fix splice update operation

[DerekBum]

Splice update operation (:) accepts 5 and only 5 arguments. It was parsed and encoded incorrectly with only 3 argument (as every other update operations). Also fixed the same operation for the crud.

Op struct made private. Change all Upsert and Update requests to accept *tarantool.Operations as ops parameters instead of the interface{}.

I didn't forget about (remove if it is not applicable):

• [x] Tests (see documentation for a testing package)
• [x] Changelog (see documentation for changelog format)
• [x] Documentation (see documentation for documentation style guide)

Closes #348