Investigate removing dependency on Guava

[GCHQDeveloper314]

Koryphe has a dependency on Guava, but the Guava methods used are simple and most functionality can be achieved with Java 8 built in methods.
Having a dependency on Guava makes Koryphe more difficult to use as a library due to version conflicts. While this can be mitigated with JAR shading, it would be better to not have the dependency at all.

Investigate removing the dependency on Guava entirely, or at least removing it from core code and using it as only a test dependency.

[GCHQDeveloper314]

It might be better to revisit this after moving entirely to Java 11.