Rust: generate test code from schema docstrings

[redsun82]

This generates test source files from code blocks in class docstrings.

By default the test code is generated as is, but it can optionally:

• be wrapped in a function providing an adequate context using @rust.doc_test_signaure(sig), with sig containing everything following the function name
• be skipped altogether using @rust.skip_doc_test

So for example an annotation like

@rust.doc_test_signature("<'a, T: Eq>(x: &'a T, y: &'a T) -> &'a T")
class Foo:
    """
    Bla bla, for example:
    ```
    if x == y { x } else { y }
    ```
    """
    ...

will result in the following wrapper:

fn test_foo<'a, T: Eq>(x: &'a T, y: &'a T) -> &'a T {
    // Bla bla, for example:
    if x == y { x } else { y }
}

[aibaars]

LGTM, thanks!