Rename the template_for header.

C++26 (I hope) is supposed to have this syntax:

```
template for( const auto &element: aggregate )
{
	...;
}
```

Thus, I've adjusted this gadget to have a similar name, to enable
simple mechanical code changes.  From Alepha, you'd use it
thus:

```
template_for( aggregate ) <=[&]( const auto &element )
{
	...;
};
```

template_for.test/CMakeLists.txt

@@ -0,0 +1 @@
+unit_test( 0 )