Add type_of as an easier way to spell CPP2_TYPEOF

I've been meaning to do this for over a year, it's time

I'd prefer `typeof`, but that would conflict with C++ compilers that implement C `typeof` in C++ mode

Author: hsutter

docs/cpp2/common.md

@@ -105,7 +105,7 @@ main: () = {
 ```
 
 
-## <a id="keywords"></a> Reserved keywords
+## <a id="keywords"></a> Contextual keywords
 
 Cpp2 has very few globally reserved keywords; nearly all keywords are contextual, where they have their special meaning when they appear in a particular place in the grammar. For example:
 
@@ -115,6 +115,8 @@ Cpp2 has very few globally reserved keywords; nearly all keywords are contextual
 
 - `type` can be used as an ordinary name (e.g., `std::common_type<T1,T2>::type`).
 
+- Unqualified `type_of(x)` is a synonym for Cpp1 `std::remove_cvref_t<decltype(x)>`.
+
 In rare cases, usually when consuming code written in other languages, you may need to write a name that is a reserved keyword. The way to do that is to prefix it with `__identifer__`, which treats it as an ordinary identifier (without the prefix).
 
 

source/to_cpp1.h

@@ -1679,6 +1679,9 @@ class cppfront
         {
             printer.print_cpp2("cpp2_"+n.to_string(), pos);
         }
+        else if (n == "type_of") {
+            printer.print_cpp2("CPP2_TYPEOF", pos);
+        }
         else {
             printer.print_cpp2(n, pos, true);
         }