Revert "Fixed big_int_impl"

The "static const int default_allocated_precision = 64;" change looks suspicious, to a point where it could be a security vulnerability. Please can we have an explanation why it was changed.

This code has not changed in over a year. Two weeks ago, it compiled successfully in GCC and CLang.

(Note this code is based upon: original MPI library (not  NSS (Netscape Security Suite) directly, because of license restrictions) with some modifications