feature-gated `System::refresh_hotpatch` with no default impl is inconvenient for crate authors

[urben1680]

When the user uses the hotpatching feature, the System trait has one additional method, refresh_hotpatch.

However, this method has no default impl. This means that other 3rd party crates the user relies on need to offer that feature too if these manually implement System, even if hotpatching makes no sense for them. If they don't do this, the user's project wont compile.

A solution could be to add an empty body as the default impl.

[alice-i-cecile]

I think an empty body default impl with a strong doc comment that "this will not work for hotpatching" is the right way to go here.