NIH plugs

Because of course we need to remake everything from scratch!

This is a work in progress JUCE-lite-lite written in Rust to do some experiments with. The idea is to have a statefull but simple plugin API without too much magic, while also cutting unnecessary ceremony wherever possible. Since this is not meant for general use (yet), the plugin API is limited to the functionality I needed, and I'll expose more functionality as I need it. See the doc comment in the Plugin trait for an incomplete list of missing functionality.