Function uniffi_bindgen::guess_crate_root

pub fn guess_crate_root(udl_file: &Utf8Path) -> Result<&Utf8Path>

Guess the root directory of the crate from the path of its UDL file.

For now, we assume that the UDL file is in ./src/something.udl relative to the crate root. We might consider something more sophisticated in future.