Resolves #3997.

There has been a desire by library authors to generalize over the types _ <: Has[_] and Any to describe conceptually "the set of all environmental requirements that can be combined generically".

This PR changes the encoding of Has.AreHas, which describes the capability to combine two types to form their intersection, from:

  trait AreHas[-R, -R1] {
    def union[R0 <: R, R00 <: R1: Tag](r: R0, r1: R00): R0 with R00

to:

  trait Union[R, R1] {
    def union(r: R, r1: R1): R with R1
  }

Instances are then provided for combinations of Has with Has, Has with Any, Any with Has, and Any with Any.

The biggest disadvantage of this proposal I can see is we lose the contravariance on the type parameters of R and R1. Previously, we could say that if R and R1 are subtypes of Has, then R0 <: R and R00 <: R1 are also subtypes of Has and so can be combined generically. But now if R is Any it is not the case that we can combine subtypes of R generically. This requires type annotations in the implementation of a couple of additional ZLayer combinators, but it only required one change to the API (adding an additional ROut1 >: ROut parameter in +!+ for variance) and no additional type annotations in any of the tests, so I think the impact will be limited to library authors who are writing their own combinators to combine environment types generically.

We don't gain a lot from this capability in our own library but it avoids the need for libraries like zio-grpc to implement it themselves and potentially provides them with a common way to do so.

Copying @thesamet.