Investigate alternatives to caffeine cache key AnyRef type constraint

[lewisjkl]

The Caffeine cache builder requires that the key and value types of the cache extend Object. In Scala this is analogous to AnyRef. To make scalacache compatible, I've added a type constraint on the key (K <: AnyRef). The strange thing is that this only seems to be necessary on Scala 2.12 and not on 2.13 or 3.

[ronnnnnnnnnnnnn]

I think this probably has to do with this issue fixed in scala-2.13. As for a solution here, I think the only option is to wrap the key type when its a value type in order for it to compile in scala-2.12. This can be done either at the point of use (like the change by @lewisjkl in the tests), or alternatively if we don't want the user to need to deal with it we can maybe try to find a way for the wrapping to be done automatically for value types (I guess that might be possible using macros maybe?)