Fix for test/parallel/taskPar/sungeun/private.chpl

This test reasons about taskIDs directly (as do others, likely), so it
required functionality like the cast operation between taskIDs and
ints.  For that reason, and since these operations are cheaper than
the task table overall, I moved them into a new internal module called
ChapelTaskID that is used by ChapelStandard, making them available to
all programs, whether or not ChapelTaskTable is enabled via
--task-tracking.