Add RE2::MatchData#deconstruct

GitHub: #57

So that RE2::MatchData can be used with Ruby's pattern matching
(https://docs.ruby-lang.org/en/3.0/syntax/pattern_matching_rdoc.html),
implement a deconstruct method that returns only the capturing groups.

This enables the following example:

    case RE2('(\d)(\d)?').match("1")
    in x, y
      puts "Matched #{x} #{y}"
    end

ext/re2/re2.cc

@@ -687,6 +687,46 @@ static VALUE re2_matchdata_inspect(VALUE self) {
   return result;
 }
 
+/*
+ * Returns the array of submatches for pattern matching.
+ *
+ * @return [Array<String, nil>] the array of submatches
+ * @example
+ *   m = RE2::Regexp.new('(\d+)').match("bob 123")
+ *   m.deconstruct    #=> ["123"]
+ *
+ *   case RE2::Regexp.new('(\d+) (\d+)').match("bob 123 456")
+ *   in x, y
+ *     puts "Matched #{x} #{y}"
+ *   else
+ *     puts "Unrecognised match"
+ *   end
+ */
+static VALUE re2_matchdata_deconstruct(VALUE self) {
+  int i;
+  re2_matchdata *m;
+  re2_pattern *p;
+  re2::StringPiece *match;
+  VALUE array;
+
+  Data_Get_Struct(self, re2_matchdata, m);
+  Data_Get_Struct(m->regexp, re2_pattern, p);
+
+  array = rb_ary_new2(m->number_of_matches - 1);
+  for (i = 1; i < m->number_of_matches; i++) {
+    match = &m->matches[i];
+
+    if (match->empty()) {
+      rb_ary_push(array, Qnil);
+    } else {
+      rb_ary_push(array, ENCODED_STR_NEW(match->data(), match->size(),
+            p->pattern->options().encoding() == RE2::Options::EncodingUTF8 ? "UTF-8" : "ISO-8859-1"));
+    }
+  }
+
+  return array;
+}
+
 /*
  * Returns a new RE2 object with a compiled version of
  * +pattern+ stored inside. Equivalent to +RE2.new+.

spec/re2/match_data_spec.rb

@@ -241,4 +241,18 @@
       expect(md.end(:foo)).to be_nil
     end
   end
+
+  describe "#deconstruct" do
+    it "returns all capturing groups" do
+      md = RE2::Regexp.new('w(o)(o)').match('woo')
+
+      expect(md.deconstruct).to eq(['o', 'o'])
+    end
+
+    it "includes optional capturing groups as nil" do
+      md = RE2::Regexp.new('w(.)(.)(.)?').match('woo')
+
+      expect(md.deconstruct).to eq(['o', 'o', nil])
+    end
+  end
 end