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
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
Merge branch 'experimental/write-file-hashed' into refs/top-bases/experimental/marc

* experimental/write-file-hashed:
  Rename "attribute sets" to "sets"

Conflicts:
	src/libexpr/eval-inline.hh
	src/libexpr/eval.cc
Merge branch 'experimental/debug-coercions-as-xml' into refs/top-bases/experimental/marc

* experimental/debug-coercions-as-xml:

Conflicts:
	src/libexpr/eval.cc
Merge commit 'refs/top-bases/experimental/marc' into experimental/marc

* commit 'refs/top-bases/experimental/marc':
  Rename "attribute sets" to "sets"