Migrating some high-entropy HTTP headers to Client Hints.

Hey folks,

Section 9.7 of RFC7231 <https://tools.ietf.org/html/rfc7231#section-9.7>
rightly notes that some of the content negotiation headers user agents
deliver in HTTP requests create substantial fingerprinting surface. I think
it would be beneficial if we took steps to reduce their prevalence on the
wire, and Client Hints looks like a reasonable infrastructure on top of
which to build.

`User-Agent` and `Accept-Language` seem like particularly tasty and
low-hanging fruit, and I've sketched out two proposals as proofs of concept:

*   `User-Agent` could be represented as ~four distinct hints: `UA`,
`Model`, `Platform`, and `Arch`: https://github.com/mikewest/ua-client-hints is
a high-level explainer, and
https://tools.ietf.org/html/draft-west-ua-client-hints a sketchy ID for the
new headers.

*   `Accept-Language` could be represented as a `Lang` hint:
https://github.com/mikewest/lang-client-hint is a high-level explainer,
https://tools.ietf.org/html/draft-west-lang-client-hint an equally sketchy
ID for the new header.

I'd appreciate y'all's feedback. Thanks!

-mike