Optimizations for very long numbers:
- gcd: recursive Euklid-Lehmer or Binary-Weber algorithms,
- sinh, cosh: use exp instead of coshsinh routines, with increased precision
  if necessary. Should bring 30% speedup for N >= 100.
- cl_F_atanhx.cc, cl_F_atanx.cc, cl_F_expx.cc, cl_F_lnx.cc, cl_LF_eulerconst.cc:
  increase speed of power series computation by use of fixed-point
  arithmetic (integers).
- Gamma function for rational x.

