Consistent whitespace for the Storable ChangeLog.
Remove pre-5.004 support code from Storable.

Yes, really, there was still some there.
Bump to a dev version number, as we're going to need a dev release to validate
the upcoming refactoring is safe.
Inline is_{stor,retriev}ing and last_op_in_netorder into their only callers.

These three Storable "predicate" functions were now each only being used in
one place. The is simpler if they are inlined.
In Storable.xs, inline mbuf2sv into its only caller.
In Storable, use newSVpvn() instead of newSVpv() as the length is known.
In Storable.xs, inline the macro dSTCXT_PTR into its only user.
In Storable.xs, add a no-op definition of dSTCXT_SV for non-ithreads builds.
In Storable.xs, inline INIT_STCXT as both definitions are now identical.

Also tidy up the multiplicity definition of SET_STCXT.
In Storable.xs, simplify and inline the macro STORE_HASH_SORT.

For 5.6.x and earlier, the interpreter context saving/restoring is not needed
prior to calling qsort(), as within the thread there is only one interpreter
in play.

Moreover, the SAVESPTR(orig_perl); in the removed code is not just pointless,
it's technically undefined behaviour. All that the SAVESPTR() macro will do
is ensure that the value of *(&orig_perl) is restored at scope exit. It
won't actually restore the interpreter context - that would need a call
to PERL_SET_CONTEXT with the value of origperl. The undefined behaviour is
because the LEAVE; which causes the scope pop action happens outside the
block, hence orig_perl has gone out of scope, and the address &orig_perl
(saved on the scope stack, written to by LEAVE), now points to invalid
memory. It is, however, quite hard to even get gcc's ASAN to trap this.

This code is likely unreachable anyway. It's only compiled in for 5.6.x and
earlier with USE_ITHREADS. ithreads was not useful until 5.8.0.