Rework cas_integer interface to be OO instead of functional.

4 jobs from parallel_for in 3 minutes 53 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #2968
build_cmake

00:47

 
  Test
passed #2969
run_tests

00:44

 
  Sanitizer
passed #2971
run_address_sanitizer

01:21

passed #2970
run_thread_sanitizer

01:00