[new_pymtl] clean up BitStruct implementation
[new_pymtl] clean up BitStruct implementation

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