api: impl Default for RegexSet

This is justified by the fact that a RegexSet is, after all, a set. And
a set has a very obvious default value: the empty set. Plus, this is
exactly what you get by passing a default `Vec` or an empty iterator to
the `RegexSet::new` constructor.

We specifically do not add a `Default` impl for Regex because it has no
obvious default value.

Fixes #905, Closes #906

src/re_set.rs

@@ -289,6 +289,12 @@ impl RegexSet {
     }
 }
 
+impl Default for RegexSet {
+    fn default() -> Self {
+        RegexSet::empty()
+    }
+}
+
 /// A set of matches returned by a regex set.
 #[derive(Clone, Debug)]
 pub struct SetMatches {

tests/set.rs

@@ -65,3 +65,10 @@ fn len_and_empty() {
     assert_eq!(not_empty.len(), 2);
     assert!(!not_empty.is_empty());
 }
+
+#[test]
+fn default_set_is_empty() {
+    let set: regex::bytes::RegexSet = Default::default();
+    assert_eq!(set.len(), 0);
+    assert!(set.is_empty());
+}