Implement all opcodes

[crsaracco]

This is an issue to track the TODO bullet point in the README.md that I removed in #94.

The following might be an incomplete list.

Plugin opcodes

• ProcessEvents
• SetSpeakerArrangement
• GetParamInfo
• GetMidiProgramName
• GetCurrentMidiProgram
• GetMidiProgramCategory
• HasMidiProgramsChanged
• GetMidiKeyName
• GetSpeakerArrangement
• SetPanLaw
• BeginLoadBank
• BeginLoadPreset

Host opcodes

• UpdateDisplay

[benbot]

Does anyone know which opcodes are missing?
If there was a list that was tracked somewhere it'd be a lot easier for new contributors to start helping.

[crsaracco]

I went through plugin.rs and host.rs and put any opcodes that had explicit TODOs in the first post. I'm not sure of each of the opcode's actual statuses, so at a bare minimum it requires removing the TODO if everything's already done.

I'll do another pass some time later and see if any of them are secretly unimplemented without a TODO.

I feel like it would be reckless of me here if I didn't remind people: do not reference any material incompatible with our MIT license -- including any VST2.4 SDKs you might have lying around.