gettuple: handle upvals also, fixes nbody

the tuple can be an upval also, not just a local.

change example/nbody.pn to use the fast tuple syntax.
time: 0m22.584s => 0m22.091s with example/nbody.pn 1000000
  = 2% win only
perl:   0m23.446s
lua:    0m7.689s
luajit: 0m0.299s