Document that this can contain either a string for expansion, or a
literal PathBuf not for expansion.
Rename the `from_path` method to `new_literal`: a very important
difference is whether it gets expanded - less important than the Rust
type. Also, now it takes `Into<PathBuf>`, which avoids a needless
clone.
(We don't change the API in `arti-client` because
`&tempfile::Tempdir()` doesn't implement `Into<PathBuf>`, so
`arti-client` has to have some new `as_ref` calls.)
Provide accessors `as_unexpanded_str` and `as_literal_path`. The
deserialisation already makes this part of the stable API,l so not
pvoding accessors seems just obstructive. They are useful for tests,
too.
Add tests for the new entrypoints, and for deserialisation of both
variants from TOML (via config, or directly) and JSON.
ed970310e2