math/big: use array instead of slice for deBruijn lookups

math/big: use array instead of slice for deBruijn lookups

This allows the compiler to remove a bounds check.

math/big/nat.go:681: index bounds check elided
math/big/nat.go:683: index bounds check elided

Change-Id: Ieecb89ec5e988761b06764bd671672015cd58e9d
Reviewed-on: https://go-review.googlesource.com/26663
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>