WIP: Top-level polymorphic schema definition

[jdegoes]

Since we are using SchemaModule, we may want to create a top-level, polymorphic definition of a schema. Something like this:

trait Schema[A] {
  def apply(m: SchemaModule): m.Schema[A]
}

Then a user can write:

object PersonSchema extends Schema[Person] {
  def apply(m: SchemaModule): m.Schema[A] = {
    import m._

    // Use everything in `m`
  }
}

If now we had a way to abstract over the primitives required by a schema, it would then be possible to have polymorphic schemas (across, e.g., Scala, JSON, XML, etc.).

[vil1]

This is (indirectly) addressed by #27

[vil1]

This was solved/made irrelevant by #27