sql: rework code generation for DDL routine

As far as new opcodes to operate on system spaces have been introduced,
there is no more need to open cursors on such tables, when it comes to
insert/delete operations. In addition, it allows to get rid of system
spaces lookups from SQL data dictionary.  Moreover, previously DDL
relied on nested parse to make deletions from system space. But during
nested parse it is impossible to convert space id to space pointer (i.e.
emit OP_SIDtoPtr opcode) without any overhead or "hacks".  So, nested
parse has been replaced with hardcoded sequences of opcodes,
implementing the same logic.

Closes #3252