v2.3.0: Proper hex check in fromString, `-preview=in` change

This new release comes with two minor changes:

• fromString will now properly check for hex characters, not just ASCII;
  This prevent a potential DoS when used with Vibe.d (user input could trigger an assert otherwise).
• toString sink overload now takes its argument via in (as opposed to scope const as previously);

The latter change only impact users who use -preview=in.