HPCC-8744 Optimize dictionary lookups of single fixed length fields

Optimise the most common dictionary lookup cases.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
HPCC-8744 Optimize dictionary lookups of single fixed length fields

Added stringN to list of optimized cases, and other minor cleanup.

Signed-off-by: Richard Chapman <rchapman@hpccsystems.com>
Merge pull request #3970 from richardkchapman/dict-field-opt

HPCC-8744 Optimize dictionary lookups of single fixed length fields

Reviewed-by: Gavin Halliday <ghalliday@hpccsystems.com>