Add Request and RequestData APIs

[sethmlarson]

Opening this branch instead of #151 mostly due to working out of my fork and not using more CI than needed.

I've responded to the comments and moved to a much simpler interface for RequestData that is mostly a Content-Type attached to a byte iterator factory.

Currently Request and RequestData aren't connected in any way, I'm not sure if they need to be if they mostly live within the scope of Session.request() anyways. We never present a Request to a user with a body on it.

Also another note here is that our interface now only presents URL and Headers, even when calling request.headers = {} you'll receive a Headers object from x = request.headers

[sethmlarson]

Closing these API PRs in favor of a scratch branch.