Abolish maint/readme and use doc include

Fixes #603 (closed).

The tests still run.

Indeed, with this change, code blocks in in README.md are interpreted precisely the same was as in lib.rs. Previously, cargo readme would add some text annotations etc.

So it is necessary to explicitly declare that in README.md now, rather than relying on cargo readme to adjust it.

Furthermore, this discrepancy does in fact seem to have confused some of us. There were a number of tests that look like they were always intended to compile, or even run, but which were being ignored. I have fixed those, apart from the one in tor-rtmock which looked nontrivial.

And, I did a preparatory run of maint/readmes to see what discrepancies there were and there was one grammar error in a src/lib.rs, which cargo readme wanted to propagate to README.md. In this MR, the flow is in the opposite direction, so we fix that too.