1.0.6.14: slightly more efficient FLONUM-TO-DIGITS

1.0.6.14: slightly more efficient FLONUM-TO-DIGITS

 * Implement SB-IMPL::WITH-PUSH-CHAR: an efficient way of constructing
   shortish strings one character at a time.

 * Use it instead of a string with fill-pointer in FLONUM-TO-DIGITS:
   Speeds up naive float printing tests by ~10%, and causes them to
   cons approx. 7% less.