Add const to begin() accessor of ranget

It was previously impossible to iterate over a `const` range, because
you couldn't call `.begin()` on it. This fixes that issue. Changing this
to `const` does not allow the internals of the range to be mutated, as
it returns a copy of the begin iterator, not a reference to it.

Author: thomasspriggs

src/util/range.h

@@ -365,7 +365,7 @@ struct ranget final
     return begin_value == end_value;
   }
 
-  iteratort begin()
+  iteratort begin() const
   {
     return begin_value;
   }