godoc: correctly categorize interface methods, performance tuning
godoc: correctly categorize interface methods, performance tuning

- interface methods appeared under VarDecl in search results
  (long-standing TODO)

- don't walk parts of AST which contain no indexable material
  (minor performance tuning)

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/6228047

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