strconv: simplify code for binary exponent float format

strconv: simplify code for binary exponent float format

Use optimized formatBits function to format mantissa and exponent.
Add benchmark for binary exponent float format.

on darwin/386

benchmark                         old ns/op     new ns/op     delta
BenchmarkAppendFloatBinaryExp     520           122           -76.54%

on darwin/amd64

benchmark                         old ns/op     new ns/op     delta
BenchmarkAppendFloatBinaryExp     76.9          84.3          +9.62%

Change-Id: If543552f1960e1655bed3a4130914e5eaa3aac69
Reviewed-on: https://go-review.googlesource.com/5600
Reviewed-by: Robert Griesemer <gri@golang.org>