Roughly 2x speedup for the internal bitLen function in arith.go.  Added TestWordBitLen test.

Performance differences against the new version of
bitLen generic:

x86-64 Macbook pro (current tip):

benchmark                old ns/op    new ns/op    delta
big.BenchmarkBitLen0             6            4  -37.40%
big.BenchmarkBitLen1             6            2  -51.79%
big.BenchmarkBitLen2             6            2  -65.04%
big.BenchmarkBitLen3             6            2  -66.10%
big.BenchmarkBitLen4             6            2  -60.96%
big.BenchmarkBitLen5             6            2  -55.80%
big.BenchmarkBitLen8             6            2  -56.19%
big.BenchmarkBitLen9             6            2  -64.73%
big.BenchmarkBitLen16            7            2  -68.84%
big.BenchmarkBitLen17            6            2  -67.11%
big.BenchmarkBitLen31            7            2  -61.57%

386 Intel Atom (current tip):
benchmark                old ns/op    new ns/op    delta
big.BenchmarkBitLen0            23           20  -13.04%
big.BenchmarkBitLen1            23           20  -14.77%
big.BenchmarkBitLen2            24           20  -19.28%
big.BenchmarkBitLen3            25           20  -21.57%
big.BenchmarkBitLen4            24           20  -16.94%
big.BenchmarkBitLen5            25           20  -20.78%
big.BenchmarkBitLen8            24           20  -19.28%
big.BenchmarkBitLen9            25           20  -20.47%
big.BenchmarkBitLen16           26           20  -23.37%
big.BenchmarkBitLen17           26           20  -25.09%
big.BenchmarkBitLen31           32           20  -35.51%

ARM v5 SheevaPlug, previous weekly patched with bitLen:
benchmark                old ns/op    new ns/op    delta
big.BenchmarkBitLen0            50           29  -41.73%
big.BenchmarkBitLen1            51           29  -42.75%
big.BenchmarkBitLen2            59           29  -50.08%
big.BenchmarkBitLen3            60           29  -50.75%
big.BenchmarkBitLen4            59           29  -50.08%
big.BenchmarkBitLen5            60           29  -50.75%
big.BenchmarkBitLen8            59           29  -50.08%
big.BenchmarkBitLen9            60           29  -50.75%
big.BenchmarkBitLen16           69           29  -57.35%
big.BenchmarkBitLen17           70           29  -57.89%
big.BenchmarkBitLen31           95           29  -69.07%

R=golang-dev, minux.ma, gri
CC=golang-dev
https://golang.org/cl/5574054