This expands mixins to allow them on any shape type.

One other thing that has been done is allowing shapes to re-define members. This could allow us to continue to require that lists / sets / maps always define their members. So instead of

list A {
    @required
    member: String
}

list B with A {}

You'd have

list A {
    @required
    member: String
}

list B with A {
    // This inherits the @required trait from A$member
    member: String
}

Trait merging works precisely like it does for the shapes themselves, which removes the need to have a bunch of awkward apply statements everywhere. The one restriction is that the target has to be the same shape.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.