[pymtl] bugfix datatypes.BitStruct __str__ copy
[pymtl] bugfix datatypes.BitStruct __str__ copy

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