Move SVt_BIND to be the lowest type after SVt_NULL. This will force all

Move SVt_BIND to be the lowest type after SVt_NULL. This will force all
code attempting to upgrade a BIND to anything into sv_upgrade(), which
for now will croak, but in future can DTRT, for whatever TRT is decided
to be. Make SvOK() check the flags of the referenant for a BIND, as I
envisage that the only flag bit that will get set on a BIND is SVf_UTF8
even if the referant has a defined value.

p4raw-id: //depot/perl@29642