builder: parallelize font instantiation

[m4rc1e]

The gftools builder has the ability to use the fontTools instancer in order to generate static fonts. I recently ran it on a family which had 360 static fonts. It took so long I could've flown to Berlin and back. Implementing this feature should be fairly trivial since it is a self contained task so I could just use the Python multiprocessing lib.

cc @davelab6

[simoncozens]

This is kinda the ninja builder ->#569.