Relax type constraint for QuicklensMapAt

[LMnet]

I relaxed a type constraint for implicit class QuicklensMapAt. Previously it works only on subtypes of the Map type. With this PR it will work for any type with the instance of QuicklensMapAtFunctor typeclass.

Real-world use-case: newtype wrapping Map. It may not have subtype relationships but could work like a Map. In my codebase, I have a custom NonEmptyMap, for example.

Relaxing type constraint leads to an implicit resolution ambiguity: compiler can't choose between QuicklensFunctor and QuicklensMapAtFunctor for each calls. To fix this I moved implicit class QuicklensEach into the LowPriorityImplicits trait.

Overall it is a safe change in terms of API. All old code will work without changes.

[adamw]

Thank you! Could you maybe add a test, which would somehow verify that the new functionality works?

I'd also add a comment as to why the .each in a low-priority implicit, as it might not be obvious looking at the code.

[LMnet]

@adamw done.

During the test implementation, I noticed that in the scala 3 code QuicklensMapAt is renamed to QuicklensIndexedFunctor and it is not affected by the original problem. So I have to create a separate scala-2 test folder to test my addition.

[adamw]

Should be released soon in 1.8.0, thanks! :)