Probably many bugs.

Known bugs:

* The overflows/underflows are not yet implemented in all functions.

* Possible integer overflows on some machines.

* mpfr_set_d may give wrong results on some architectures.

Potential bugs:

* get_str.c: condition len != n + 1 is always satisfied at the end.
  This means that there are too many reallocations (possibly useless
  ones) and a possible bug.