Free functions can be unconditionally compiled into the .o file, but
conditionally hidden in the header file. Static class methods don't have that
flexibility: without a declaration in the header file, you can't compile a
function definition in the .cpp file. That makes it awkward to use the same
llcommon build for production and for unit tests.
Why make the function declarations conditional at all? These are debugging
functions. They break the abstraction, they peek under the covers. Production
code should not use them. Making them conditional on an #ifdef symbol in the
unit-test source file means the compiler would reject any use by production
code. Put differently, it allows us to assert with confidence that only unit
tests do use them.
Put new free functions in (lowercase) llsd namespace so as not to clutter
global namespace.
Tweak the one known consumer (llsd_new_tut.cpp) accordingly.