checker: add an interface check for mutability, fixes #1081, fixes #7038 (#11963)

vlib/io/readerwriter.v

@@ -3,6 +3,7 @@ module io
 // ReaderWriter represents a stream that can be read from and wrote to
 pub interface ReaderWriter {
 	// from Reader
+mut:
 	read(mut buf []byte) ?int
 	// from Writer
 	write(buf []byte) ?int
@@ -11,8 +12,8 @@ pub interface ReaderWriter {
 // ReaderWriterImpl is a ReaderWriter that can be made from
 // a seperate reader and writer (see fn make_readerwriter)
 struct ReaderWriterImpl {
-	r Reader
 mut:
+	r Reader
 	w Writer
 }