math/big: replace local versions of bitLen, nlz with math/bits versions

Verified that BenchmarkBitLen time went down from 2.25 ns/op to 0.65 ns/op
an a 2.3 GHz Intel Core i7, before removing that benchmark (now covered by
math/bits benchmarks).

Change-Id: I3890bb7d1889e95b9a94bd68f0bdf06f1885adeb
Reviewed-on: https://go-review.googlesource.com/38464


Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>