mutable decl syntax change

examples/programs/io/main.bro

@@ -85,7 +85,7 @@ bad_writer func() io.Writer {
 }
 
 rejects_bad_read func() bool {
-	buffer [1]mut u8 = [0]
+	buffer [1]mut u8 := [0]
 	_ = io.read(bad_reader(), buffer[..]) catch |err| {
 		return err == .read_failed
 	}
@@ -108,7 +108,7 @@ rejects_bad_write func() bool {
 
 main func(init process.Init) i32 {
 	system io.Io :: init.io
-	buffer [2]mut u8 = [0, 0]
+	buffer [2]mut u8 := [0, 0]
 	count usize :: io.read(ok_reader(), buffer[..]) catch 0
 	if count != 2 or buffer[0] != 'o' or buffer[1] != 'k' {
 		return 1