Macro tvix_eval::value::gen_cast

macro_rules! gen_cast {
    ( $name:ident, $type:ty, $expected:expr, $variant:pat, $result:expr ) => { ... };
}

Generate an as_* method returning a reference to the expected type, or a type error. This only works for types that implement Copy, as returning a reference to an inner thunk value is not possible. Generate an as_*/to_* accessor method that returns either the expected type, or a type error.