Log of integer to base 1/n can have exact numeric results

[Ralf Stephan]

Log of integer to base 1/n can have nonsymbolic results but this is not recognized in Integer::log():

sage: ZZ(8).log(1/2)
log(8)/log(1/2)

nor in Integer.exact_log():

sage: ZZ(8).exact_log(1/2)
...
TypeError: no conversion of this rational to integer

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​de1acfa

21517: handle ZZ.log(1/n)

[Travis Scrimshaw]

I would avoid doing the exact_log twice:

        if type(m) == Integer and type(self) == Integer:
             elog = self.exact_log(m):
             if m**elog == self:
                 return elog

Similarly for the other one. Otherwise LGTM.

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​4aba8a9

21517: avoid duplicate computations

[Ralf Stephan]

Thanks.