Migration to 2021 Edition

[wwylele]

For a proc macro crate, migration to 2021 Edition is important, because the code wrapped inside the html macro will follow macro crate's edition, instead of parent project's edition. This in turn will affect e.g. resolution for array iterator, which can appear quite often.

The internal of typed-html isn't trivial to migrate to 2021 Editon, unfortunately, because it heavily uses dyn-less trait object that's obscured by internal macro. I have made a dirty patch for my own use wwylele@ec33ad5