Add serde Serialize/Deserialize derives to common settings like Value, Decibels, Mapping, etc

[BrianWiz]

This is something highly desired on my end because I've built up a data-driven workflow where I define my sound emitters in hot-reloaded .RON files. However right now I have to map from my types to Kira types.

[tesselode]

Decibels and Mapping already have serde implementations if you enable the serde feature. Value doesn't because it can contain ModulatorIds, which aren't (de)serializable because they don't mean the same thing across multiple runs of a program.

[tesselode]

If you find something that should be serializable but isn't, let me know!