`ruint-macro/src/lib`: BLOCKED) Replace with Diagnostic API when stable.

*On 2022-05-28 @recmo wrote in [`1ec2e50`](https://github.com/recmo/uint/commit/1ec2e50003548890260e8c06886f44fd826bc07b) “Recognize new tags”:*

BLOCKED) Replace with Diagnostic API when stable.
See

```rust
);
TokenStream::from_str(&source).unwrap()
}

/// Construct a compiler error message.
// FEATURE: (BLOCKED) Replace with Diagnostic API when stable.
// See
fn error(span: Span, message: &str) -> TokenStream {
// See: https://docs.rs/syn/1.0.70/src/syn/error.rs.html#243
TokenStream::from_iter(vec![
TokenTree::Ident(Ident::new("compile_error", span)),
TokenTree::Punct({

```
*From [`ruint-macro/src/lib.rs:24`](https://github.com/recmo/uint/blob/1ec2e50003548890260e8c06886f44fd826bc07b/ruint-macro/src/lib.rs#L24)*