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/write-file-hashed' into experimental/write-file-hashed

* commit 'refs/top-bases/experimental/write-file-hashed':
  Rename "attribute sets" to "sets"

Conflicts:
	src/libexpr/primops.cc