WIP: VST3 wrapper

[m-hilgendorf]

Work in progress VST3 wrapper.

Notes:

• The VST3 wrapper is entirely a macro instead of an internal struct. This requires making WrappedPlugin public
• Still need to work out how I/O negotiation should work, VST3 doesn't rely on channel count. Failing a few unit tests in the VST3 Validator
• Parameter string functions left unimplemented
• Added normalize/denormalize as functions to WrappedPlugin.
• Blocked on #35 in vst3-sys, Cargo.toml points to my fork.

The code is dense and could probably be cleaned up in a few places.