mat/big: add raw access to Int bits
mat/big: add raw access to Int bits

This is a minimal API extension, it makes it possible
to implement missing Int functionality externally w/o
compromising efficiency. It is the hope that this will
reduce the number of feature requests going directly
into the big package.

Also: Fixed some naming inconsistencies: The receiver
is only called z when it is also the result.

R=golang-dev, agl
CC=golang-dev
http://codereview.appspot.com/5607055

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