Assign Arc params without enabling params menu

[mimetaur]

Create public methods to set and clear assignments directly via code. Decouple core functionality from Norns param system.

[mimetaur]

Expert users should be able to arbitrarily map and remap encoders without being routed through the Norns param system.

For an example use case see https://llllllll.co/t/arcify/22133/7?u=mimetaur