C: clean wrong PERL_UNUSED_VAR(x), x is used
C: remove unneeded GV* tmpgv and SV* tmpsv

removed with 1.43
C 5.6: skip undefined 5.6 API: PL_threadhook, PerlIO_destruct, my_curse

but 5.6 B::C still broken (regression)
C: move PerlIO_destruct before our global destruction

and more minor harmonizations
1.45_08: Replace each %hash with foreach keys (#307)

In B::C, Bytecode and CC.
The big issues were only hash traversal (B::HV::hwalk) in Bytecode
and B::walksymtable in B::C
CC: keep each %$declare
each-hash: skip new B::C::walksymtable also
de-randomize all foreach keys to sort

we better have predictable and comparable hash key ordering (syms, packages, ...)