region: add a utility class for keeping track of regions of some range

region: add a utility class for keeping track of regions of some range

This is basically like the range_map stuff in src/base (range already
exists in Python).  This code is like a set of ranges.  I'm using it
to keep track of changed lines in source code, but it could be use to
keep track of memory ranges and holes in memory regions.  It could
also be used in memory allocation type stuff.  (Though it's not at all
optimized.)