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/lexer.lpp

@@ -51,7 +51,7 @@ EQUAL                "="
 EQUAL_EQUAL          "=="
 FALSE                "false"
 FN                   "fn"
-FNTY                 "fnty"
+FN_TYPE              "__Fn"
 IF                   "if"
 IMPL                 "impl"
 IMPORT               "import"
@@ -144,7 +144,7 @@ string_literal        \"([^\\\"\n\v\f\r]|\\.)*\"
 {EQUAL_EQUAL}         { return SIMPLE_TOKEN(EQUAL_EQUAL);         }
 {EQUAL}               { return SIMPLE_TOKEN(EQUAL);               }
 {FALSE}               { return SIMPLE_TOKEN(FALSE);               }
-{FNTY}                { return SIMPLE_TOKEN(FNTY);                }
+{FN_TYPE}             { return SIMPLE_TOKEN(FN_TYPE);             }
 {FN}                  { return SIMPLE_TOKEN(FN);                  }
 {IF}                  { return SIMPLE_TOKEN(IF);                  }
 {IMPL}                { return SIMPLE_TOKEN(IMPL);                }

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,