message: api: add 'empty' constructor to RegexSet
  This goes nicely with the new 'is_empty' predicate introduced in the
  previous commit. It's a bit of a nicer way of building a set with no
  regexes instead of the weird `&[""; 0]` or something even stranger since
  the RegexSet::new constructor is polymorphic.
  Closes
diff: @@ -96,6 +96,19 @@ impl RegexSet {
  RegexSetBuilder::new(exprs).build()
  }
  + /// Create a new empty regex set.
  + ///
  + /// # Example
  + ///
  + /// ```rust
  + /// # use regex::RegexSet;
  + /// let set = RegexSet::empty();
  + /// assert!(set.is_empty());
  + /// ```
  + pub fn empty() -> RegexSet {
  + RegexSetBuilder::new(&[""; 0]).build().unwrap()
  + }
  +
  /// Returns true if and only if one of the regexes in this set matches
  /// the text given.
  ///
  