First order transformations

Really, really, really important: no more discrete selectors and transformation at the lowest level.

So, for example, e/set-attr would be a transformation on its own, without a selector.

Then one would use apply-to (I just made the name) to create a new transformation from e/set-attr and a selector.

This would allow (sample-for [selector] [for-comprehension] ...) a better clone-for.