Refactoring the OP_Column opcode for improved performance and
maintainability.
Further performance tweaks to OP_Column.
Improved comments on the OP_Column changes.  Optimize out loading of overflow
pages for content with zero length.  Add test cases for the latter.
Reduce the size of the VdbeCursor object from 144 to 120 bytes.
Unpack some fields, adding some space back to the VdbeCursor object,
in order to help the code to run a little faster.
Reduce the size of VdbeCursor again, this time without a performance hit.
Refactor the OP_Column opcode to make it clearer and easier to maintain.
Overall, performance tests show about a 1% speed increase with this change.
Performance optimization to the OP_Next and OP_Prev opcodes.
A simple change to the OP_Function opcode improves overall performance by
about 0.5%.
Another improvement to OP_Function and an improvement to OP_Move.