Refactor Module -> Pallet

[kianenigma]

The term module, used a lot in FRAME is historically linked to SRML.

Not a big deal, and probably lots of people are used to the current naming, but it is technically wrong. Now the term module is replaced with pallet. Hence,

1. decl_module! -> decl_pallet!.
2. struct Module -> struct Pallet.

I think both can be addressed by #5678 cc @thiolliere

[bkchr]

I don't think that we should touch any of the old code.

You can bring in your ideas in the proc-macro conversion pr, but we should not break the old code. Let it stay and we will deprecate it anyway with the merge of the new proc-macro macros.

[kianenigma]

Yes I think the PR that closes #5678 should also close this.

[bkchr]

Sounds like we can close this issue directly? There is no real reason to have 100 issues open for one pr. If you want something specific, I propose that you comment on the original issue or directly in the pr.