Factored out `self_replace` from rustup

[simonsan]

Can be found here: https://docs.rs/self-replace/latest/self_replace/

Maybe it would be nice to share that code within self_update?

[mitsuhiko]

I was looking into this. The benefit would be that currently this crate leaves leftovers behind after the update: https://github.com/jaemk/self_update/blob/1a507bb4cd687d14e044dce8b7b2d66846ea581c/src/update.rs#L147-L150