Make Utf8CStr a first class citizen in C++ codebase

Utf8CStr is in many cases a better string view class than
std::string_view, because it provides "view" access to a string buffer
that is guaranteed to be null terminated. It also has the additional
benefit of being UTF-8 verified and can seemlessly cross FFI boundaries.

We would want to start use more Utf8CStr in our existing C++ codebase.

native/src/base/cstr.rs

@@ -369,7 +369,7 @@ impl AsRef<Utf8CStr> for Utf8CStr {
 
 // Notice that we only implement ExternType on Utf8CStr *reference*
 unsafe impl ExternType for &Utf8CStr {
-    type Id = type_id!("rust::Utf8CStr");
+    type Id = type_id!("Utf8CStr");
     type Kind = cxx::kind::Trivial;
 }