Use step rather than [] for dictionaries

src/preserves.nim

@@ -240,7 +240,7 @@ proc excl*(pr: var Preserve; key: Preserve) =
       delete(pr.set, i, i)
       break
 
-proc `[]`*(pr, key: Preserve): Preserve =
+proc `[]`*(pr, key: Preserve): Preserve {.deprecated: "use step instead".} =
   ## Select a value by `key` from `pr`.
   ## Works for sequences, records, and dictionaries.
   if pr.isDictionary:
@@ -940,8 +940,10 @@ proc fromPreserve*[T, E](v: var T; pr: Preserve[E]): bool =
       if tupleLen(T) ==  pr.len:
         result = true
         for key, val in fieldPairs(v):
-          try: result = fromPreserve(val, pr[toSymbol(key, E)])
-          except KeyError: result = false
+          let pv = step(pr, toSymbol(key, E))
+          result =
+            if pv.isSome: fromPreserve(val, get pv)
+            else: false
           if not result: break
     else: discard
   elif T is ref: