Implement/fix things needed for multis.

Handle QAST::BVal, compile the only child of the CompUnit.
Fix nqp::setcodeobj,nqp::getcodeobj.
Implement nqp::captureposelems,nqp::captureposarg,nqp::usecapture,nqp::savecapture.
Implement nqp::curcode. Pass test 75 for that.