sql. Make _space and _index accessible to SQL

sql. Add OP_ParseSchema2.

OP_ParseSchema adds new objects to the schema. A typical VDBE CREATE
program creates objects like BTree-s, updates sqlite-master and then
updates the schema with this opcode.

Originally, OP_ParseSchema fetched data from sqlite_master. Since we're
about to remove sqlite_master, OP_ParseSchema2 was created. The later
opcode perfoms similar to OP_ParseSchema; it extracts data from a range
of VDBE registers.