Can also lower $_.

Before, this earned us a SEGV in the Rakudo setting compile, but now
that issue is fixed, we can do this optimization.
Optimize simple 'for' loops.

Can avoid a lot of block invocations in some cases, making things a
bit cheaper.
Added nqp::seekfh on JVM.
Added nqp::seekfh, nqp::readfh on parrot backend.
Add tests for seekfh.
Fix seekfh failures on JVM backend.
Add elems to P6opaque.
fix test plan, nwc10++
bump moar rev for bigint fixes
bump moar rev for num->str fix
bump moar rev for to unbreak test that fail under spesh
Merge branch 'master' of github.com:perl6/nqp into loop_labels