Fix Xcode 6 compile errors relating to tut::ensure_equals() overloads.

lltut.h declares a number of ensure_equals() overloads for various data types,
notably the types supported by LLSD. We expect these to be called by tut code.
But the tut code in question is in a template in tut.hpp -- which was
#included BEFORE the overloads were declared. Previous C++ compilers have
evidently made multiple passes, collecting the relevant overloads before
attempting to compile the template bodies. clang does not, complaining that
the overloads must be declared before the tut.hpp template code that
references them. Reordering parts of lltut.h seems to address that problem.
For similar reasons, test programs that use StringVec.h and its operator<<()
must #include StringVec.h before lltut.h.
Add ensure_equals(const std::string&, const LLSD::Binary&, const LLSD::Binary&)
overload. The sloppy mix of (const char*, ...) and (const std::string&, ...)
overloads bothers me, since for many of those ... types we seem to have to
duplicate them.