feat(string): add Debug for Regex

Manual @debug.Debug impl renders as <Regex: <pattern>>.

Author: bobzhang

string/pkg.generated.mbti

@@ -34,6 +34,7 @@ pub struct Regex {
 
   fn new(StringView) -> Regex raise
 }
+pub impl @debug.Debug for Regex
 pub fn Regex::capture(Self, String) -> Self
 pub fn Regex::execute(Self, StringView, last_index? : Int) -> MatchResult?
 pub fn Regex::find(Self, StringView) -> Iter[MatchResult]

string/regex.mbt

@@ -27,6 +27,11 @@ const MAX_REPEAT_QUANTIFIER = 256
 ///|
 using @debug {trait Debug, type Repr}
 
+///|
+pub impl Debug for Regex with to_repr(self) {
+  Repr::opaque_("Regex", @debug.to_repr(self.pat))
+}
+
 ///|
 /// Result of one successful match produced by `Regex::execute`.
 struct MatchResult {