Re-remove dependency on Guava

[john-h-kastner-aws]

Requires the following changes:

• Use copyOf and unmodifiableSet functions to replace ImmutableSet. Prefer using copyOf because it creates an unmodifiable copy of the set or list, so mutation to the input does not reflect in the new object. unmodifiableSet can still be used to return an immutable view of a set without causing any copying.

• Roll our own memoizing supplier. This change should be reviewed by someone with more Java knowledge, but, AFAIK, the behavior should be the same.

Issue #, if available:

86

Description of changes:

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[andrewmwells-amazon]

Maybe ask Lucas to review?

[exceptionfactory]

@john-h-kastner-aws Raising the question, is this change planned for reconsideration at some point? It would be helpful to remove the 3 MB dependency on Google Guava for those integrating with the Cedar Java library.