review(ImplicitArray): general reorganization improvements

- Put `ImplicitBackends` into its own module
- Improve documentation in multiple areas
- Add more details to change log
- Remove some commented code in testing
- Simplify some cmake variables and logic
- Remove the set warnings on implicit arrays
- Use the `string(CONCAT ...)` instead of internal function when
  creating dispatch array list
- Add copyright guards to tests that were missing them
- Remove some exporting for MSC 2008/2010 compilers
- Add some `static_assert`s to testing