Add isRecord(…) with arity descriminator

src/preserves.nim

@@ -411,6 +411,12 @@ func isRecord*(pr: Preserve; label: string): bool {.inline.} =
     pr.record.len > 0 and
     pr.label.isSymbol(label)
 
+func isRecord*(pr: Preserve; label: string; arity: Natural): bool {.inline.} =
+  ## Check if ``pr`` is a Preserves record with the given label symbol and field arity.
+  pr.kind == pkRecord and
+    pr.record.len == succ(arity) and
+    pr.label.isSymbol(label)
+
 proc isSequence*(pr: Preserve): bool {.inline.} = pr.kind == pkSequence
   ## Check if ``pr`` is a Preserves sequence.