Add unembed routine

src/preserves.nim

@@ -382,6 +382,12 @@ iterator fields*(pr: Preserve): Preserve =
   ## Iterate the fields of a record value.
   for i in 0..<pr.record.high: yield pr.record[i]
 
+proc unembed*[E](pr: Preserve[E]): E =
+  ## Unembed an `E` value from a `Preserve[E]` value.
+  if pr.kind != pkEmbedded:
+    raise newException(ValueError, "not an embedded value")
+  pr.embed
+
 proc writeVarint(s: Stream; n: int) =
   var n = n
   while true: