* Split the code.

* Restructure.

* Split tests.

* Self-review.

* Break lines.

* Move tests out.

* Rename CheckEra -> CheckMortality but keep backwards compatibility

* Update frame/system/src/extensions/check_mortality.rs

* Don't rename the IDENTIFIER for now.

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