drnim: tiny progress (#13882)

* drnim: tiny progress
* refactoring complete
* drnim: prove .ensures annotations
* Moved code around to avoid code duplication
* drnim: first implementation of the 'old' property
* drnim: be precise about the assignment statement
* first implementation of --assumeUnique
* progress on forall/exists handling

compiler/ast.nim

@@ -218,7 +218,7 @@ type
     nkEnumFieldDef,       # `ident = expr` in an enumeration
     nkArgList,            # argument list
     nkPattern,            # a special pattern; used for matching
-    nkHiddenTryStmt,      # token used for interpretation
+    nkHiddenTryStmt,      # a hidden try statement
     nkClosure,            # (prc, env)-pair (internally used for code gen)
     nkGotoState,          # used for the state machine (for iterators)
     nkState,              # give a label to a code section (for iterators)