Add and use modular cps.

Also, a new subdir "lib" is added, it helps keeping track
of "non-master"/"feature" cps files:

* lib/point-child-base.cps: reference point sub-elements of another master as "base"
* lib/weight.cps: change the distance of the left/right on-curve points from the center
* lib/scale.cps: scale width and height of the skeleton
* lib/single-inheritance.cps: create a master by referencing another master
* lib/interpolate-2.cps: create a interpolated master from 2 sources by interpolating the outlines
* metapolate-2.cps: create a master from 2 sources by interpolating the CPS

There are also masters using cases for all of it.