Redo makefile to inject version, fix snapshots, update docs

[StevenACoffman]

I broke this out of #127 so this is my first stacked diff.

• Update snapshots to make tests pass
• Redo Makefile and inject version into building/installing
• Update documentation for injecting version into go run-ing
• Add makefile niceties like make help and advice from https://tech.davis-hansson.com/p/make/

This presumes that we want to continue to maintain a Makefile. If we want to use Mage I would be more than happy to do that instead, as Mage would probably work better on a windows machine that didn't have WSL installed.

[csilvers]

I'll wait to see if ben wants to comment on this -- in general I'm happy to keep ben as the main maintainer if he wants to be! I'm not sure what thoughts he had.

I will point out that this doesn't do anything for our use at Khan, which runs genqlient using go run. In general, I wonder if we should just go with the simpler solution of hard-coding the version number in the library, and using that. It's probably already hard-coded somewhere.

[StevenACoffman]

@benjaminjkraft What are your thoughts?

[benjaminjkraft]

Perhaps I misunderstood -- you could explain more in the PR message -- but this seems very incomplete since the recommended way to run genqlient is go run? I don't think telling people to do //go:generate go run ... -ldflags ... really worth it, I think we should either fix the runtime.BuildInfo method or just put a version const in the source.

[csilvers]

(Just to get off my queue)

[dnerdy]

(Also requesting changes to get this PR off my queue.)

[benjaminjkraft]

1.18 adds simpler ways to do this!