    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>

bdwalton/debian-golang,tianon/debian-golang,pombredanne/go-deleteme,dsriram/golang,bdwalton/debian-golang,pombredanne/go-deleteme,bdwalton/debian-golang,tianon/debian-golang,tv42/old-go,apparentlymart/golang,apparentlymart/golang,bdwalton/debian-golang,ssrl/go,gnanderson/go,dsriram/golang,dsriram/golang,ssrl/go,gnanderson/go,h8liu/golang,h8liu/golang,gnanderson/go,pombredanne/go-deleteme,nightlyone/golang,ssrl/go,h8liu/golang,ssrl/go,ssrl/go,tv42/old-go,apparentlymart/golang,tv42/old-go,gnanderson/go,tv42/old-go,nightlyone/golang,tianon/debian-golang,tianon/debian-golang,bdwalton/debian-golang,gnanderson/go,dsriram/golang,h8liu/golang,tv42/old-go,h8liu/golang,tianon/debian-golang,apparentlymart/golang,nightlyone/golang,nightlyone/golang,pombredanne/go-deleteme,apparentlymart/golang,bdwalton/debian-golang,pombredanne/go-deleteme,h8liu/golang,dsriram/golang,dsriram/golang,nightlyone/golang,gnanderson/go,ssrl/go,pombredanne/go-deleteme,tv42/old-go,nightlyone/golang,bdwalton/debian-golang,h8liu/golang,tianon/debian-golang,dsriram/golang,apparentlymart/golang,bdwalton/debian-golang,gnanderson/go,tv42/old-go,tianon/debian-golang,nightlyone/golang,tianon/debian-golang,ssrl/go,pombredanne/go-deleteme,apparentlymart/golang