chore: Replace generational-arena with slotmap

[torokati44]

Because generational-arena is unmaintained, and slotmap is the recommended alternative.

Most of it was a trivial search-and-replace, except for:

• That .rev() from LoadManager is gone, because this iterator type does not support it.
  • Note that iteration order was undefined before, and is undefined now. Therefore, we shouldn't have been relying on this already.
  • Replaced by a clumsy hand-cranked reversing solution, but it should behave the same (undefined, but accidentally usually correct) way.
• loader.rs has sometimes used Index instead of the Handle type it defines, in its API.
  • We should probably also rename that to match the convention of the rest of the handle types.
• The dummy handle in the socket tests was replaced by the default, I hope that's fine, @kjarosh.

In the future, we could consider:

• Using more specialized implementations for faster iteration
  • https://docs.rs/slotmap/latest/slotmap/index.html#choosing-slotmap-hopslotmap-or-denseslotmap
• Defining different key types for the different handles, as is the proper intended usage
  • Instead of using DefaultKey everywhere
• Introducing a named key/handle type (or alias, for now) in desktop/executor.rs and web/lib.rs as well.

[torokati44]

I'll try to ping @relrelb due to the iteration order thingy...