pub fn read<P: AsRef<Path>>(path: P) -> io::Result<Vec<u8>> {
    fn inner(path: &Path) -> io::Result<Vec<u8>> {
      let mut file = File::open(path)?;
      let mut bytes = Vec::new();
      file.read_to_end(&mut bytes)?;
      Ok(bytes)
    }
    inner(path.as_ref())
  }

Reference implementations must NOT have abstractions like this. Rust encourages it. Lots of Rust codebase is filled with them. Your feelings for Rust is irrelevant. C is simple and easy to understand, therefore reference implementations must be in C. Period.

...or Common Lisp, or OCaml... why not?!

(a) that is a fairly easy to understand piece of code. Are you complaining about the definition of the inner function?

(b) the equivalent C code would look pretty similar.

(c) this is not cryptographic code