1.0.6.19: optimize BIGNUM-TRUNCATE'ing by small powers of two

1.0.6.19: optimize BIGNUM-TRUNCATE'ing by small powers of two

	* This is a common case when printing floating-point numbers.  On
	  the simple "print a million single-floats" benchmark, this wins
	  by about 20-25%;
	* Also fold a few i+1 loop variables into their only use; doing so
	  is not much worse that what we had before and slightly better if
	  the backend supports DATA-VECTOR-REF-WITH-OFFSET.