add build-time ETag to /-/static/* responses & return "304 Not Modified"

[syphar]

The first step of #1560, extracted from #3006.

Can save quite a bit of transfer time for slow connections, but also saves time between CDN & origin. Of course, not specifically for these files, it's just the ones I started with.

Generally I want to try not to use the often "simple" etag impl that would aways generate the full html, then MD5-Hash it, and only then comparing it with If-None-Match. Better is to work with an ETag that can be generated without fetching the actual content.

Next steps after this:

• use / reuse the ETag the S3 API already gives us, to add ETag / If-None-Match to rustdoc asset handlers (toolchain & invocation)
• generate an ETag for rustdoc html pages, my current hunch is that we could implement that by using

[GuillaumeGomez]

We keep adding more deps. T_T

Build time (from clean) on my computer is getting in high numbers.