refactor(delegation): move impl Pallet into pallet mod

[weichweich]

No Ticket

Move the Pallet impl block into the pallet module like we do it for all other pallets as well.

Checklist:

• I have verified that the code works
  • No panics! (checked arithmetic ops, no indexing array[3] use get(3), ...)
• I have verified that the code is easy to understand
  • If not, I have left a well-balanced amount of inline comments
• I have left the code in a better state
• I have documented the changes (where applicable)

[wischli]

LGTM