[type traits] Provide replacement for deprecated classes AlwaysFalse, AlwaysTrue

The classes AlwaysFalse, AlwaysTrue reside in the deprecated header
dune/common/static_assert, yet they are frequently used to document
template code (see FS#1435 for details). As the C++11 standard library
does not provide a straight forward replacement for theses classes, we
offer C++11 style reimplementations. The replacements to_false_type,
to_true_type reside in the namespace Std and are thus easily
identifiable as Dune specifis.