gc: walk pointer in range on slice/array
gc: walk pointer in range on slice/array

R=ken2
http://codereview.appspot.com/166071

npe9/noahevans-go9,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,bryanxu/go-zh,scirelli/scirelli-go,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,webfd/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,d0f/go-zh,mhennings/marcohennings-go,Triskite/willstone-goclone,d0f/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,rflanagan/reginaldflanagan-project1,Triskite/willstone-goclone,webfd/go-zh,d0f/go-zh,Triskite/willstone-goclone,Triskite/willstone-goclone,d0f/go-zh,rflanagan/reginaldflanagan-project1,npe9/noahevans-go9,npe9/noahevans-go9,sanjosh/sanjos100-tipc,scirelli/scirelli-go,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,rflanagan/reginaldflanagan-project1,mhennings/marcohennings-go,bryanxu/go-zh,webfd/go-zh,Triskite/willstone-goclone,bryanxu/go-zh,npe9/noahevans-go9,rflanagan/reginaldflanagan-project1,webfd/go-zh,bryanxu/go-zh,Triskite/willstone-goclone,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,Triskite/willstone-goclone,Triskite/willstone-goclone,d0f/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,d0f/go-zh,scirelli/scirelli-go,mhennings/marcohennings-go,rdp/rogerpack2005-golang,scirelli/scirelli-go,rflanagan/reginaldflanagan-project1,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,npe9/noahevans-go9,sanjosh/sanjos100-tipc,bryanxu/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,scirelli/scirelli-go,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,npe9/noahevans-go9,webfd/go-zh,rflanagan/reginaldflanagan-project1,bryanxu/go-zh,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof