* Added the constexpr specifier to the placeholder_transform_t constructor, alog with tests - to confirm replace_placeholders, and other supplied transforms, are now working at compile time.
* Added macro guards to avoid new tests which fail under MSVC. Also added the second string argument to the static asserts to maintain C++14.
* Removing tests which cause problems for older versions of GCC.
* Scaled the tests back to work with GCC 6.
* ...and of course C++14-style static asserts.
* Fixed an bug in placeholder_transform_t::operator() which used a non-existent value member of rvalue_mover. This allows replacements() and replace_placeholders() to be used. Also added a test file which checks these two, as well as evaluation().
* Unlike the evaluation transform, the placeholder transform returns an expression, and so should work with print. A print test was also added.
