Continue adding test coverage for the new data dictionary: ALTER INDEX.

Debug index:alter().
Add tests for changes in index id, name, type, unique/not unique.
Make box.space[i].index[j] output yaml-parsable (- idx: index 0
parsing problem).
Continue covering MODIFY INDEX with tests.

Implement tuple validation when building a new index.
This introduces such checks as for field type, presence of a field,
etc. in all tuples added to the new index.

Add error code ER_INDEX_ARITY, update tests.

Add a check that the primary key is always unique.

Test unique -> non-unique transition.