Extend convert-constants support for dicts

This enhances convert-constants to not flatten dicts completely, but
also generate a so-called association list for them.

This allows either direct use of the 'lookup' function, or (for
performance) conversion to Data.Map and optimised lookup later.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>