pallet-staking: Reorg migration, pallet and pallet impls

[emostov]

closes #9408

The main thing left out of this is moving the storage items and types to their own module (they are left in lib.rs for now). I think that should be easy to do in a follow up PR and leaving it out now will make it slightly easier to merge this change into other open PRs that touch staking.

Note, the visibility on few functions in impl.rs was increased from either private or pub(super) to pub(crate).

cc @kianenigma

[emostov]

@kianenigma I am thinking for now we can just move the migrations module. And then once the bags PR becomes very stable we can do a bigger re-org. My concern with doing a big re-org now is that transferring those changes to the bags PR would be difficult and may result in us losing some details without realizing.

[kianenigma]

I am fine with both. The bags PR has a small footpring in the rest of the pallet, so I think it is possible to do the full move as well. Nonetheless, this is not worth risking anything. We can always do this later.

[kianenigma]

types.rs?

[emostov]

Yeah I was thinking something like that for a follow up

[kianenigma]

Add license header and an appropriate //! single line of module doc to each new file, and I am all fine with this change.

[shawntabrizi]

don't really see the need for the pallet folder, rather than a simple lib.rs file for that stuff, but okay.

[emostov]

bot merge

[ghost]

Trying merge.