• fix bug #21747: Re: weirdity in bignum... (powers and high precision):
  infinite loops for blog() (and consequently bpow()) if you requested
  an accuracy greater than 67 digits (uses _log() now, and not blog())
  Thanx to darconc!
  
• cache the result of _log(2) and _log(10) so that subsequent calculations
  can re-use the already done work
  
• instead of computing _log(10), compute _log(1.25) and _log(2) and then do:
  _log(1.25 * 2 * 2 * 2) = _log(1.25) + _log(2) + _log(2) + _log(2)
  This makes computing _log(10) much faster, so that computing blog(N) is
  about a factor of 5 faster when N >= 10 or N <= 0.1
  
• add bexp()