Extend trim::CallSet with a new range-based add method.

Previously, individual calls had to be added one-at-a-time. This was
adequate for trimming functionality where one call is examined at a
time.

But I'm now wanting to use this same CallSet code to dramatically
optimize the performance of callset specifications on the apitrace
command-line, (in particular large callsets resulting from "apitrace
trim --print-callset"). In this case, we really want to add entire
call ranges rather than just one call at a time.