[PATCH] switch to the single-header Rapid YAML, upgrade to v0.5.0 Rapid YAML has an extensive CMake based build, which is probably ok but might be a pain 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.
