encode-name: fix 305 for threaded perl

with threads a svop const is really a padop
assoc some more special SV syms, fixes #306 $/

  $\ => PL_ors_sv
  $/ => PL_rs
  $@ => PL_errors

This list might not be complete yet and I'm unsure if local $/ to other
values than undef will work.
Added local $/ testcase to t/issue256.t.
See #256 for tracking all such special symbols.
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