out parameters: enforce that 'out' is only used as a parameter (#20510)

* out parameters: enforce that 'out' is only used as a parameter

* make tables.nim use 'out' parameters

* better backwards compat

lib/core/macros.nim

@@ -75,7 +75,7 @@ type
     nnkTupleTy, nnkTupleClassTy, nnkTypeClassTy, nnkStaticTy,
     nnkRecList, nnkRecCase, nnkRecWhen,
     nnkRefTy, nnkPtrTy, nnkVarTy,
-    nnkConstTy, nnkMutableTy,
+    nnkConstTy, nnkOutTy,
     nnkDistinctTy,
     nnkProcTy,
     nnkIteratorTy,         # iterator type
@@ -125,6 +125,9 @@ type
 
   TNimSymKinds* {.deprecated.} = set[NimSymKind]
 
+const
+  nnkMutableTy* {.deprecated.} = nnkOutTy
+
 type
   NimIdent* {.deprecated.} = object of RootObj
     ## Represents a Nim identifier in the AST. **Note**: This is only