Consider whether const generics supersedes the `Token!` macro

Something like:

```rust
pub struct Token;
```

Then you'd use `Token<'+'>` for what used to be `Token![+]`, and `Token<'+', '='>` instead of `Token![+=]`, etc.

The `custom_punctuation!` macro (https://docs.rs/syn/1.0.105/syn/macro.custom_punctuation.html) would no longer be necessary because you could instantiate a `Token` with any arbitrary punctuation without needing to give it a name.