Storable: add SEEN0 macro

to shut up a compiler warning.

The SEEN(y,stash,i) macro expands to something like

    ...;
    if (stash)
       foo_NN(stash);

where foo_NN is a function that expects a non-null arg.

SEEN() is sometimes called as SEEN(sv,0,0) which of course expands to

    if (0)
       foo_NN(0);

which under gcc at least, emits the 'non-null' warning before the entire
block of code is optimised away.

So add a SEEN0() macro which handles the stash=0 case.
Storable: crash on ref to blessed tied array

When Storable was retrieving a tied array, if that array needed blessing
into a class, the code was passing the name of the class, rather than the
HV of the stash, to sv_bless(), causing a crash.

(Discovered due to a gcc "var set but not used" warning).

I also updated a few source code comments with s/SX_FOO/SX_TIED_FOO/.
Storable: silence some unused var warnings
Storable: silence compiler 'unused func' warnings

Two static functions are only used within asserts, so only define them
if asserts are enabled
toLOWER_LC(), toUPPER_LC(): fix signedness

The are documented to return UV, but in one definition they return
tolower()/toupper(), which on Linux return a signed value. So
cast away the compiler warnings.
APItest.xs: fix various compiler warnings