Implement Generics

[ayyaruq]

Requires Go 1.18 beta 1, but I dislike beta releases. Schedule looks like 1.18 is released in Feb 2022 tho, so we can get started and test at least. The Frame and Message decoders can all be ported to generics, along with Subscribers as they're basically fulfilling an interface but otherwise identical.

Note that this should use type constraints, as the syntax without constraints is kinda gross, and constraints make it easier to add more decoders in the future.