Convert Preserves sets to JSON arrays

A one way conversion but still useful.

src/preserves/jsonhooks.nim

@@ -56,6 +56,14 @@ proc fromPreserveHook*[E](js: var JsonNode; prs: Preserve[E]): bool =
     for i, val in prs.sequence:
       if not fromPreserve(js.elems[i], val):
         return false
+  of pkSet:
+    js = newJArray()
+    js.elems.setLen(prs.set.len)
+    var i: int
+    for val in prs.set:
+      if not fromPreserve(js.elems[i], val):
+        return false
+      inc i
   of pkDictionary:
     js = newJObject()
     for (key, val) in prs.dict.items: