Upgrade Rapid YAML to v0.5.0, and switch to the single file amalgamation

[johnbartholomew]

Rapid YAML has an extensive CMake based build, which is probably ok but may be awkward to use from our other build systems (e.g., Bazel, which we probably want to continue supporting).

However, Rapid YAML also provides a single-file amalgamation (aka unity build), which is of course much easier to embed.

I expect this makes it harder for users (e.g., distribution packagers) to use a system-provided Rapid YAML install instead. I'd like to fix that, though I'm not sure if anyone is doing that currently - unlike the json lib we don't seem to have prior support for it in the CMake build at least. So it can probably be done as a follow-up. But ideally before the next published release.

[johnbartholomew]

A few open bugs should be fixed just by upgrading Rapid YAML:

• #942 (which has a contributed but not-yet-merged test case #943)
• #1014
• #1050

YAML-parsing-related bug #1109 is not fixed by the upgrade and needs other work.