add some expect_false/expect_true hints
unroll sv_setiv(sv,x) where SvTYPE(sv) == SVt_NULL

We use this pattern a lot, and the normal code path does a lot
of unnecessary work in the SVt_NULL case, so we can speed stuff
up (a lot!) if we unroll this case. This involves gratuitious
intimacy with the internals.
fixup redefine issues in perl-5.10
only do fast-iv on new perls

needs a define as some of these structures dont exist in older perls.
make it possible to use parts of bench without installing tons of serialization modules