Irmin_pack_unix.RangesSourceAn ordered set of disjoint (offset, length) ranges.
add ~off ~len t inserts the range (off, len) into t. When add is called multiple times sequentially, it is optimized for strictly decreasing offsets arguments.
iter fn t calls fn ~off ~len on every disjoint range (off, len) in the set t. The function fn ~off ~len is called with strictly increasing offsets. If two or more consecutive ranges (off,len) and (off+len,len') were added to the set t, a single call to fn will be performed on the englobing interval (off,len+len').