strconv: use bounded bits.TrailingZeros instead of shifts table

strconv: use bounded bits.TrailingZeros instead of shifts table

The strconv shifts table is 320 bytes (amd64) and is present in
many binaries since integer formatting is very common.

Instead of using a precalculated table with shift amounts
use a bounded bits.TrailingZeros to determine the shift amount
to format numbers in a base that is a power of 2.

amd64:
name        old time/op  new time/op  delta
AppendUint   379ns ± 1%   286ns ± 2%  -24.62%  (p=0.000 n=20+19)

Change-Id: Ib94d9b033321b41e975868943c7fcd9428c5111e
Reviewed-on: https://go-review.googlesource.com/110478
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>