Meta-Issue for improvements to `@compact`

[avik-pal]

• [ ] #588 for specifying constants directly
• [ ] Special syntax for providing functions that are evaluated in initialparameters (maybe @initfn annotation)
• [ ] (Very Speculative) Rewrite Lux Layers with @compact for readability. (This is going to be very breaking for how parameters and states are arranged currently)
  • To clarify confusion, the container layers in Lux are NOT GOING ANYWHERE; the change, if ever done, will be an internal implementation detail, mostly invisible to the end user.
• [ ] Write a document on how compact works. It would clarify problems like https://github.com/LuxDL/Lux.jl/issues/516.
• [x] ~https://github.com/LuxDL/Lux.jl/issues/692~