Restore Storable's DEBUGME build after commit 591596833b093b3c

Storable.xs can conditionally compile debugging code if the C pre-processor
macro DEBUGME is defined. By default, it is not.

Commit 591596833b093b3c changed the name and purpose of the second argument
to the macro BLESS(), but missed updating the name in one location, within
code conditionally enabled by DEBUGME, hence breaking compilation with
DEBUGME.

Fix compilation by correcting that code to use the new macro parameter name.
Storable should not assume that sizeof(mg_len) is 4.

Commit 6174b39a88cd4874 changed mg_len from I32 to SSize_t. sizeof(I32) is 4
everywhere (except certain Crays, for which Storable has work-around code).
In the version object serialisation code, Storable was passing mg->len
directly to the macro WLEN(), and and it turns out that some paths through
this macro is relying on the assumption that the value passed in is 32 bits.
This is now invalid on on 64 bit systems, but only triggered an error with
the existing tests on big endian systems.

The easiest fix is to assign the value to a temporary variable of the
correct size, and process that. However, a lot of the code makes a lot of
unwarranted assumptions about sizeof(int), and ideally should be audited and
rewritten to use more appropriate types.