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
https://golang.org/cl/1227042