Add items iterator

Iterates over all child items in a Preserve.

src/preserves.nim

@@ -224,6 +224,21 @@ proc `$`*(prs: Preserve): string =
   of pkEmbedded:
     result.add(prs.embedded.repr)
 
+iterator items*(prs: Preserve): Preserve =
+  case prs.kind
+  of pkRecord:
+    for i in 0..prs.record.high.pred:
+      yield prs.record[i]
+  of pkSequence:
+    for e in prs.sequence.items: yield e
+  of pkSet:
+    for e in prs.set.items: yield e
+  of pkDictionary:
+    for k, v in prs.dict.pairs:
+      yield k # key can be an arbitrary Preserve
+      yield v
+  else: discard
+
 func isRecord*(prs: Preserve): bool =
   if prs.kind == pkRecord:
     result = true