Allow using external objects containing references

[sapir]

For example:

#[derive(runestick::Any, Default)]
struct MyObject<'a> {
    something: &'a u32,
}

Currently, runestick::Any requires std::any::Any which requires 'static, and also UnsafeFromValue and UnsafeToValue require 'static, so this is impossible. Also the derive macro doesn't handle the lifetime correctly.

It would be nice if this were allowed when not using async.