Change core libraries as needed in order to use or work well with constructor tearoffs

[eernstg]

This issue tracks support of the constructor tearoffs feature. See the enclosing meta-issue for details.

[lrhn]

There are no planned changes to the platform libraries.
We may want to use the feature, in cases where we currently eta-expand a constructor instead of tearing it off, but that's neither urgent nor important and can be fixed at any later time. It should not block release of the feature.