    Trivial optimization. 	Cached string indexing in inner loop of Btoui64.
    Trivial optimization.
	Cached string indexing in inner loop of Btoui64.

    Before:
    strconv_test.BenchmarkAtoi   5000000           309 ns/op
    strconv_test.BenchmarkAtoiNeg    5000000           325 ns/op
    strconv_test.BenchmarkAtoi64     5000000           465 ns/op
    strconv_test.BenchmarkAtoi64Neg  5000000           469 ns/op

    After:
    strconv_test.BenchmarkAtoi  10000000           182 ns/op
    strconv_test.BenchmarkAtoiNeg   10000000           193 ns/op
    strconv_test.BenchmarkAtoi64    10000000           251 ns/op
    strconv_test.BenchmarkAtoi64Neg 10000000           258 ns/op

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/1227042

Committer: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

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