Kernel: Add formatter function for OwnPtr<KString>

This adds a formatter function for OwnPtr<KString>. This is added mainly
because lots of dbgln() statements generate Strings (such as absolute
paths) which are only used for debugging. Instead of catching possible
OOM situations at all the dbgln() callsites, this makes it possible to
let the formatter code handle those situations by outputting "[out of
memory]" if the OwnPtr is null.

Kernel/KString.h

@@ -48,7 +48,18 @@ template<>
 struct Formatter<Kernel::KString> : Formatter<StringView> {
     void format(FormatBuilder& builder, Kernel::KString const& value)
     {
-        Formatter<StringView>::format(builder, value.characters());
+        Formatter<StringView>::format(builder, value.view());
+    }
+};
+
+template<>
+struct Formatter<OwnPtr<Kernel::KString>> : Formatter<StringView> {
+    void format(FormatBuilder& builder, OwnPtr<Kernel::KString> const& value)
+    {
+        if (value)
+            Formatter<StringView>::format(builder, value->view());
+        else
+            Formatter<StringView>::format(builder, "[out of memory]"sv);
     }
 };