Minor cleanups of equals and hashcode in core

[anuraaga]

Just a starter PR with some cleanups as I get to know the repo better :) In particular, I noticed the patterns were mixed within the module so this makes them consistent too.

Description of changes:

• Use hashCode method instead of Objects.hash. The later has poor performance because it allocates a varargs array even for this single argument

• instanceof instead of getClass() ==. Java 8+, which the module targets, performs better with instanceof, and the code is a bit cleaner to boot thanks to automatic null check. getClass could still be appropriate where a subclass really has to match a subclass, but this doesn't seem to be intended

• Object.equals instead of a tree of null checks

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[willarmiros]

Thanks for the improvements!