Small improvement to padic_log_balanced.
Small improvement to padic_log_balanced.

Note that we now reduce y modulo pN on input, and that
this is all the data that we need to take into account
in order to compute the logarithm to precision N.  The
reduction process of (t, r) ensures that only look at
relevant data.

jpflori/flint2,dsroche/flint2,jpflori/flint2,fredrik-johansson/flint2,dsroche/flint2,wbhart/flint2,fredrik-johansson/flint2,wbhart/flint2,jpflori/flint2,wbhart/flint2,dsroche/flint2,jpflori/flint2,dsroche/flint2,fredrik-johansson/flint2