As mentioned I'm not too big of a fan of public implements as it sounds wrong.
implementation or trait are probably more suited.
Regarding ConvertIndex since it's more a property/trait shouldn't it be Indexable?

Probably it's performance-wise better to introduce a typeclass with methods to write and read the type to/from the native hashtable, so it would be possible to use HashMap with native types and strings without any overhead, as they would use their own methods, like saveString, loadString, etc.

Yes, that's a good idea. Just have to figure out how to do this so that it's not necessary to implement two type classes for every type that does not have a hasmap native.

I don't think this is gonna happen anymore, closing! New generics have been updated to support common usage patterns - but type classes have been deemed overkill and will not be implemented for now.