[pymtl] hack datatypes.BitStruct to use __str__ from def
[pymtl] hack datatypes.BitStruct to use __str__ from def

tj93/pymtl,cfelton/pymtl,cornell-brg/pymtl,cfelton/pymtl,tj93/pymtl,cfelton/pymtl,12yujim/pymtl,tj93/pymtl,12yujim/pymtl,cornell-brg/pymtl,Glyfina-Fernando/pymtl,12yujim/pymtl,Glyfina-Fernando/pymtl,jjffryan/pymtl,Glyfina-Fernando/pymtl,jjffryan/pymtl,jck/pymtl,cornell-brg/pymtl,jck/pymtl,jjffryan/pymtl,jck/pymtl