fs-mistrust: Document problems with non-UTF8 OsString in toml
It turns out that the toml
crate can't handle OsString, since
toml
doesn't support serialize_newtype_variant, and the serde
crate tries to serialize OsString using that method.
In this commit we document that limitation, and test that we can at least round-trip through json.
Found by inspecting test coverage.
This supersedes !534 (closed)