Phase 7 followup: set equality in core-= + phs-to-struct

- phm.janet: add phs-to-struct (converts set to struct via phm-to-struct)
- core.janet: core-= now handles set? with phs-to-struct normalization
- test/phase7-test.janet: add (= #{1 2 3} #{3 2 1}) assertion
- 315 ok, 2 fail (pre-existing, unchanged)

test/phase7-test.janet

@@ -19,7 +19,7 @@
   (assert (= false (ct-eval ctx "(set? [1 2 3])")) "set? false")
   (assert (= 4 (ct-eval ctx "(count (conj #{1 2 3} 4))")) "conj add")
   (assert (= 2 (ct-eval ctx "(count (disj #{1 2 3} 3))")) "disj")
-  (assert (= 3 (ct-eval ctx "(count #{1 2 3})")) "count"))
-(print "  passed")
+  (assert (= 3 (ct-eval ctx "(count #{1 2 3})")) "count")
+  (assert (= true (ct-eval ctx "(= #{1 2 3} #{3 2 1})")) "= order-independent"))
 
 (print "\nAll Phase 7 tests passed!")