box: introduce tuple_format_iterator class

The similar code in tuple_field_map_create and
vy_stmt_new_surrogate_delete_raw that performs tuple decode
with tuple_format has been refactored as reusable
tuple_format_iterator class.

Being thus encapsulated, this code will be uniformly managed and
extended in the further patches in scope of multikey indexes.

Extended engine/json test with vy_stmt_new_surrogate_delete_raw
corner case test. There was no problem before this patch, but
small bug appeared during tuple_format_iterator_next
implementation was not covered.

Needed for #1257