Added Run convenience function

TODO.txt

@@ -1,7 +1,7 @@
-Add a convenient way to run parsers
 Add repetition parsers to avoid some recursion
 Think about not requiring so much Pos() when making messages
 Rename Seq2 to Seq
 Document Seq
+Should MakeState be private now that there's Run?
 Add SPDX tags
 What's Megaparsec got that we ain't got?

gigaparsec.go

@@ -165,6 +165,27 @@ func (p Parser[In, Out]) Label(label string) Parser[In, Out] {
 	}
 }
 
+type ParseError Message
+
+func (pe ParseError) Error() string {
+	return Message(pe).String()
+}
+
+func Run[In, Out any](p Parser[In, Out], c cursor.Cursor[In]) (out Out, err error) {
+	start := MakeState(c)
+	result, err := p(start)
+	if err != nil {
+		err = fmt.Errorf("Run: %w", err)
+		return
+	}
+	if failed, _, msg := result.Failed(); failed {
+		err = ParseError(msg)
+		return
+	}
+	_, _, out, _, _ = result.Succeeded()
+	return
+}
+
 // Return creates a parser that always succeeds and returns value without consuming any input.
 func Return[In, Out any](value Out) Parser[In, Out] {
 	return func(state State[In]) (Result[In, Out], error) {