sql: remove AggStep0 and OP_BuiltinFunction0

This patch moves the initialization of sql_context out of the VDBE. This
allows us to remove the opcodes OP_BuiltinFunction0 and OP_AggStep0,
which work in a rather strange way. Moreover, due to the changes these
opcodes make to the VDBEs, it is possible that the estimated size of the
VDBE may change during execution of VDBE, which could lead to various
problems.

Needed for #4145