Refactor package name from 'io.tast' to 'tast'

* Also move all actors to an 'actor' package.
A local cache lookup can return nothing
Covariant-value cache specification

* A covariant-value cache can be assigned to a cache of a supertype.
* For a covariant-value cache, a subtype-value cache can be passed
as parameter.
Switch to FunSuite in order to be able to run individual tests from IDE
Replace, if the old value is the same, implemented

* A local cache should not replace a value if old value is the same
* A local cache should not replace a value if old value is different
* A local cache should not replace a value if old value does not exist