wasmparser: `for_each_operator!` assumes specific name of the lifetime

`for_each_operator!` generates `BrTable<'a>` as an argument for one of the visitors. This in essence depends on the trait definition to specify `'a` as the lifetime generic and implement specifically `VisitOperator<'a>`. Ideally macro could figure out and implicitly use the right lifetime without requiring the user to specify `'a` specifically.

I tried a couple of things to remove this not-super-hygienic ambient-authority kinda thing:

1. Generate `BrTable<'_>` instead – rustc wants to see `'a` there;
2. Add `type BrTable = wasmparser::BrTable<'a>` to the trait declaration and pass `::BrTable` – associated type defaults are unstable.

Didn’t spend any time thinking if there are other approaches we could take here.