Car reference fs

[d70-t]

This is a potential implementation for #2.

I don't yet know what a good interface would be. Currently it can be used to generate a reference filesystem from an existing car file as follows:

from ipldstore.car_reference_fs import car2reference_fs
carfilename = "test.car"
indexfilename = "index.car"
ref = car2reference_fs(carfilename)    
with open(indexfilename, "w") as reffile:    
    json.dump(ref, reffile)  

[thewtex]

@d70-t this is awesome!! I will test and follow-up on #2