Feature proposal: Selector language

We have quite a few use cases where there is a need to select a set of types based on some criteria. A simple example of this is the `@@` augment syntax in #218 (which in the current proposal only supports selecting by identity). Other examples include type constraints, emitters and validators.

There may be benefit of providing a common way to express such selectors. I believe @bterlson may have additional use cases up his sleeve.