syntax: add 'Repetition::with'

This is useful when doing structural recursion on a '&Hir' to produce a
new 'Hir' derived from it.

Author: BurntSushi

regex-syntax/src/hir/mod.rs

@@ -1739,6 +1739,19 @@ pub struct Repetition {
     pub sub: Box<Hir>,
 }
 
+impl Repetition {
+    /// Returns a new repetition with the same `min`, `max` and `greedy`
+    /// values, but with its sub-expression replaced with the one given.
+    pub fn with(&self, sub: Hir) -> Repetition {
+        Repetition {
+            min: self.min,
+            max: self.max,
+            greedy: self.greedy,
+            sub: Box::new(sub),
+        }
+    }
+}
+
 /// A type describing the different flavors of `.`.
 ///
 /// This type is meant to be used with [`Hir::dot`], which is a convenience