Update outdated comments

exercises/036_enums2.zig

@@ -6,10 +6,10 @@
 //     const Stuff = enum(u8){ foo = 16 };
 //
 // You can get the integer out with a builtin function,
-// @enumToInt(). We'll learn about builtins properly in a later
+// @intFromEnum(). We'll learn about builtins properly in a later
 // exercise.
 //
-//     const my_stuff: u8 = @enumToInt(Stuff.foo);
+//     const my_stuff: u8 = @intFromEnum(Stuff.foo);
 //
 // Note how that built-in function starts with "@" just like the
 // @import() function we've been using.

exercises/064_builtins.zig

@@ -4,7 +4,7 @@
 // Ziglings exercise.
 //
 // We've also seen @intCast() in "016_for2.zig", "058_quiz7.zig";
-// and @enumToInt() in "036_enums2.zig".
+// and @intFromEnum() in "036_enums2.zig".
 //
 // Builtins are special because they are intrinsic to the Zig
 // language itself (as opposed to being provided in the standard

exercises/065_builtins2.zig

@@ -6,8 +6,8 @@
 //
 // ...and lots of type casting operations such as...
 //
-//      @as          @intToError   @intToFloat
-//      @intToPtr    @ptrToInt     @enumToInt
+//      @as          @errorFromInt  @floatFromInt
+//      @ptrFromInt  @intFromPtr    @intFromEnum
 //
 // Spending part of a rainy day skimming through the complete
 // list of builtins in the official Zig documentation wouldn't be