Add "!skip" command to SqlRun.
Fix https://github.com/julianhyde/optiq/issues/294, "Implement DENSE_RANK windowed aggregate function".

Changes to the `AggImplementor` and `WinAggImplementor` interfaces to provide richer context to user code implementing a (windowed) aggregate function.

RelDataType instances created via built-in type strategies are now NOW NULL. Add 'boolean nullable' parameter to RelDataTypeImpl.proto methods.

Upgrade to linq4j-0.4, and add 'final' to code-generator variables, per https://github.com/julianhyde/linq4j/issues/27.