Rename "attribute sets" to "sets"

We don't have any other kind of sets so calling them attribute sets is
unnecessarily verbose.
Merge commit 'refs/top-bases/experimental/debug-coercions-as-xml' into experimental/debug-coercions-as-xml

* commit 'refs/top-bases/experimental/debug-coercions-as-xml':
  Rename "attribute sets" to "sets"

Conflicts:
	src/libexpr/eval-inline.hh
	src/libexpr/eval.cc