HPCC-9790 Use a hash table to quickly resolve expressions

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>
Merge pull request #4710 from ghalliday/issue9790

HPCC-9790 Use a hash table to quickly resolve expressions

Reviewed-By: Richard Chapman <rchapman@hpccsystems.com>