Migrate serve command to hyper 0.13

[samford]

As discussed in #782, hyper is already included in Zola due to the reqwest dependency (used in the link_checker and templates components). Replacing Actix with hyper in the serve command reduces the number of dependencies and slightly improves build times and binary size.

Preliminary numbers on my system (macOS 10.15) are as follows:

	Actix 1.0	hyper 0.13	Change
Build Items	411	334	-23%
Build Time (debug)	2m 44s	2m 11s	-25%
Build Time (release)	9m 16s	8m 16s	-12%
Binary (debug)	87.4 MB	77.1 MB	-13%
Binary (release)	17.3 MB	16.4 MB	-5%
Binary (release, strip)	15.8 MB	15 MB	-5%

These numbers depend on your hardware and operating system but it gives you a relative idea of the improvements here. Some of the changes are modest but they don't come with any cost, so it seems like an easy win.

This is currently based on the reqwest 0.10 draft PR (#892) and I'll rebase this when reqwest 0.10 is released and that PR is merged. I wanted to get this up as a draft in the interim time, so folks can review it and we can address any implementation issues beforehand. Feel free to check out the branch, build it, and run it locally to see if you encounter any issues on your system (as I've only tested this on macOS).

[Keats]

Very nice, I'll merge it when reqwest 0.10 is released!

[samford]

This has been rebased on the reqwest v0.10 release PR, so it should be good.

As mentioned in the reqwest v0.10 PR, the site component's check_site test is still intermittently hanging until it times out. It's a coin toss as to whether it will manifest when the tests are run and, unfortunately, it appeared in this run (on linux-1.39). I can confirm that it also happens on my local setup sometimes, so it's not just CI.

[Keats]

@samford Can you rebase?

[samford]

Rebased on the current next branch now.

[Keats]

I've tried it locally and it works well, thanks a lot for that. The build time change is really noticeable!