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,

executable_semantics/testdata/function/fnty.carbon

@@ -16,6 +16,6 @@ fn add1(x: i32) -> i32 {
 }
 
 fn main() -> i32 {
-  var f: fnty(i32)->i32 = add1;
+  var f: __Fn(i32)->i32 = add1;
   return f(-1);
 }

executable_semantics/testdata/generic_function/apply.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn apply[T:! Type, U:! Type](f: fnty (T) -> U, x: T) -> U {
+fn apply[T:! Type, U:! Type](f: __Fn (T) -> U, x: T) -> U {
   return f(x);
 }
 

executable_semantics/testdata/generic_function/tuple_map.carbon

@@ -11,7 +11,7 @@
 
 package ExecutableSemanticsTest api;
 
-fn map[T:! Type](f: fnty (T) -> T, tuple: (T, T)) -> (T, T) {
+fn map[T:! Type](f: __Fn (T) -> T, tuple: (T, T)) -> (T, T) {
   return (f(tuple[0]), f(tuple[1]));
 }