Make defined methods in abstract classes final to prevent stupid things from happening.

* Checkstyle recommended this.
Suppress the varargs warnings. `Arrays.asList` is perfectly safe.
Make `equals` and `hashCode` less crazy in AbstractFixedRekord.
Avoid shadowing.
Let `Objects` take care of the Rekord hash code.
Disable Checkstyle for the weird varargs build methods.

It flags the `final` as unnecessary, which is true, but conflicts with `@SafeVarargs`' need for it.
Avoid inline conditionals.
Enable Checkstyle, with a custom configuration.