Allow pallet in construct_runtime to have fixed index (#6969)

* implement index for pallet + some tests

* add test and doc

* remove deprecated and document behavior

* update internal doc

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* address review

* use index for all module, break construct_runtime

* fix line length

* implement migration helper funciton in scheduler

* fix start at index 0

* Update frame/scheduler/src/lib.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/support/procedural/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* bump frame-metadata crate

* factorize

* avoid some unwrap and remove nightly join

* Update frame/support/src/event.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* fix test

* add test and improve error message

* factorize test

* keep iterator, and use slice instead of vec

* refactor to avoid to have expects

* small refactor

* Test something

* Make sure we update the `Cargo.lock`

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* return 2 error

* Apply suggestions from code review

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/scheduler/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* fix typo

* Revert "fix typo"

This reverts commit f2de8f2db34e8ac72bc9c34437c60dca3fa4ac22.

* Revert "Update frame/scheduler/src/lib.rs"

This reverts commit 6feb4605c6f784b64591e229de7a6fec6dbffb4b.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>