view: operator bool should be explicit

Implicit bool tends to allow some unexpected behaviour when indexing
with incorrect types. eg, `view<int const*> a; a["foo"];`

view.hpp

@@ -275,7 +275,7 @@ namespace cruft {
 
         ///--------------------------------------------------------------------
         /// Returns true if the view is not empty; ie, there is data remaining.
-        constexpr operator bool () const noexcept
+        explicit constexpr operator bool () const noexcept
         {
             return not empty ();
         }