Begin making changes to the IN operator in an attempt to make it run faster
and to make the code easier to understand.
The idea of coding IN operator with a short list on the RHS as an OR expression
turns out to be helpful.  If the list is of length 1 or 2, the OR expression
is very slightly faster, but the ephemeral table approach is clearly better for
all list lengths greater than 2.  Better to keep the code simple.
Clean up the IN operator code generation logic to make it easier to reason
about.  In the process, improve code generation to omit some unused OP_Null
operations.
Remove an unnecessary OP_Null in the IN-operator logic.  Attempt to clarify
comments explaining the IN-operator code, though it is not clear that the
comments are correct even yet - more work to be done.
Improved detection and handling of NULL values on the RHS of a IN operator.
A better comment on the generated code for the NULL-in-RHS-of-IN detection
logic.