make as_base_exp literal, like fraction

[smichr]

as_base_exp no longer does any introspection, it just returns the literal arguments of the Pow, so the return values of S.Half**x and 1/2**x are no longer the same.

>>> [i.as_base_exp() for i in (S.Half**x, 1/2**x)]
[(1/2, x), (2, -x)]

Release Notes

• core
  • as_base_exp output is now literally (self.base, self.exp)