• streamline _scale_a() and _scale_p() for more speed in rounding
  
• remove the now unnec. support for MB_NEVER_ROUND and {_f}, this
  makes all ops that call round() a tad faster (one exists is
  removed) and shrinks the codesize a bit
  
• streamline bneg(), inline is_zero(): makes it 1.6 times faster
  
• replace ref($_[0]) w/ undef when it isn't actually needed. This
  makes some ops (bsstr(), bneg etc) about 2% faster for small numbers
  
• restrict low-level math library names to sane chars to avoid
  exploitation of eval()
  
• fill_can_cache() accidentally did checks for 'or' & 'xor'
  
• inline _fill_can_cache
  
• add _register_callback() to notify subclasses of lower math lib changes
  
• bgcd() is now about 10% faster
  
• is_positive(0) == 0, since 0 is neither positive nor negative
  
• streamline bmod() a bit
  
• fix blog() constructing arguments (broke Math::BigInt::Constant)