Add Cache

[wmontwe]

For the contact refactoring, I need to use a different cache and extracted a general purpose solution. It provides a InMemoryCache, ExpiringCache and SynchronizedCache to fit several needs.

[cketti]

Since these tests are only running on the JVM we can use spaces in method names for better readability.

For assertions newer code typically uses Truth (e.g. assertThat(…).isEqualTo(…)).

[wmontwe]

assertThat

Even if Truth works with Kotlin, it is not optimized for Kotlin. What about selecting a Kotlin assertion library that also has support for Kotlin Multiplatform?

This could help to select one:

• KotlinAssertionLibraryComparison
• Write a comparison to Kluent, AssertK, Strikt, Atrium, Kotest, other Kotlin assertion libraries? · Issue #849 · google/truth

[cketti]

Aside from the duplicate test LGTM 👍