Instead of a VST3-style polling function to test if a plugin supports a
certain layout, the plugin now explicitly enumerates the supported
layouts. This aligns better with non-VST3 plugin formats.
I've been putting this off for a while, but now is finally the day. We
already did this for CLAP a while back. This is both simpler and less
error prone.
This is needed to enable sending and receiving SysEx #54. Because
associated type defaults still are not stable, this requires every
plugin that doesn't need this to set this to the unit type:
```rust
type SysExMessage = ();
```
This will make it possible to run background tasks in a type safe way.
Sadly, this does mean that every plugin now needs to define the type
alias and constructor function since Rust does not yet support defaults
for associated types.
This solves the same problem as #27 but in a cleaner way. The previous
commits refactored the smoothing to make it possible to use the
calculations in plugin code to do the same thing as #27, but then I
realized that that doesn't make much sense since the time to decay into
complete silence isn't very meaningful for this kind of computation.
Only a couple of these functions would be needed during initialization.
In the next couple commits ProcessContext will get a way to access
auxiliary IO, so this really had to be separated.
