Reimplement the Scala 3 schema derivation without Mirror

[vigoo]

Original problem:

The following does not work:

object T {
   implicit val schema: Schema[T] = DeriveSchema.gen
   val (...) = makeAccessors[T]
}

def makeAcessors[T](implicit schema: Schema[T]) = schema.makeAccessors(customAccessorBuilder)

while if we let the compiler to use the type generated by DeriveSchema it does:

object T {
   implicit val schema = DeriveSchema.gen[T]
   val (...) = makeAccessors[T]
}

The problem is that Scala 3 does not compile this, requiring all implicits to have explicit type annotation (which is the first version).

We could do the following trick to work around this:

object T {
   val generatedSchema = DeriveSchema.gen[T]
   implicit val schema: generatedSchema.type = generatedSchema
   val (...) = makeAccessors[T]
}

and this compiles on Scala 2, but it creates cyclic reference on Scala 3.

The reason for that is that summon is called before checking whether we are on top level and that is easy to fix:

            val summoned = if (!top) Expr.summon[Schema[T]] else None
            summoned match {
              case Some(schema) =>

instead of

            Expr.summon[Schema[T]] match {
              case Some(schema) if !top =>

but that still does not work because Mirror(typeRepr) returnsNone for a type like that (I guess it is also trying to summon the implicit for T under the hood).

If the macro does not get the Mirror (just generates null for example) then the code with the above workaround compiles on Scala 3, so I believe a derive macro that does not rely on Mirror would solve this issue