add etag & conditional get to rustdoc html output

[syphar]

most interesting piece for #1560 .

Using a safe approach for etag generation, without having to download or rewrite the HTML from storage.

I wasn't 100% certain about adding the dependency, let me know what you think :)

( alternative is to add serde::Serialize everywhere and using serde_json to serizlialize into the hash)

[syphar]

@GuillaumeGomez this one can be reviewed now, will safe some time between browsers & cdn, but mainly between CDN & origin.

[syphar]

additional commits LGTM