refactor: move cargo workspace to root of repo

removed rust/ and move it's contents to more proper locations.

main changes:

- pulled rust/forks/ to the root of the repo - this is a temporary
  directory anyways as these forks will be removed/replaced in the
  near(ish) future.
- moved all light-client related items to light-clients/, and renamed
  rust.nix to ethereum-light-client.nix (since that's all that file
  does)
- moved rust/protos/ to a new folder generated/rust; generated/ is
  intended to contain any and all generated code we check in to the
  repo.
- moved rust/serde-utils tot a new folder utils/rust/serde-utils;
  utils/ is intended to contain any internal utility code we may have,
  for whatever language.
- added unionvisor to the workspace.