Mark fnty as __Fn to reflect experimental state (#928)

The `__Fn` naming is intended to mirror things like `__Continuation`.

The reason for this path is because it's not clear this is the form we'll want, and I think experimental naming will help reflect that.

executable_semantics/syntax/parser.ypp

@@ -164,7 +164,7 @@
   EQUAL_EQUAL
   FALSE
   FN
-  FNTY
+  FN_TYPE
   IF
   IMPL
   IMPORT
@@ -377,7 +377,7 @@ expression:
           context.source_loc(), Operator::Ptr,
           std::vector<Nonnull<Expression*>>({$1}));
     }
-| FNTY tuple return_type
+| FN_TYPE tuple return_type
     {
       auto [return_exp, is_omitted_exp] = $3;
       $$ = arena->New<FunctionTypeLiteral>(context.source_loc(), $2, return_exp,