Add fully functional Ruby client compatible with Go client

[brandur]

Here, try to do a full port of River to Ruby using a similar technique
to Rust through repeated refinement iterations.

I also had it try to go through each of River's features sections in the
docs and make sure we have equivalent Ruby functionality.

I changed a fair bit through manual inspection of the resulting APIs,
but there's still a reasonable possibility we'll notice more things and
want to make a few additional tweaks before fully committing.

I also have a full Pro implementation locally, but have left it out of
this change set. We'd ship this from a private gem server separately.