sql: introduce opcodes to operate on system spaces

Since it is impossible to use space pointers during execution of DDL
(after any DDL schema may change and pointers fetched at compile time
can become expired), special opcodes to operate on system spaces have
been introduced: OP_SInsert and OP_SDelete. They take space id as
an argument and make space lookup each time before insertion or deletion.
However, sometimes it is still required to iterate through space (e.g.
to satisfy WHERE clause) during DDL. As far as now cursors rely on
pointers to space, it is required to convert space id to space pointer
during VDBE execution. Hence, another opcode is added: OP_SIDtoPtr.
Finally, existing opcodes, which are invoked only during DDL, have been
also refactored.

Part of #3252