Make incremental_evaluate( ) return a pair.
Return {R,R} for constants.
Return result if the result is already set.
Return result for computation of new result.
Return another result.
make RegOperationArgs::evaluate_* routines return the result.

This allows us to remove access_result in favor of simply
M.access(R3).C.
Use returned result in reg_heap::lazy_evaluate( ).
Make incremental_evaluate_in_context(,) also return a pair.
Remove some unused code.