fmt: remove math package dependency and avoid float operations
Remove floating point comparisons and rely only on the information
directly provided by appendFloat.
Make restoring the zero padding flag explicit instead of using a defer.
Rearrange some case distinctions to remove duplicated code.
Add more test cases for zero padded floating point numbers with sign.
benchmark old ns/op new ns/op delta
BenchmarkSprintfFloat-4 187 180 -3.74%
Change-Id: Ifa2ae85257909f40b1b18118c92b516933271729
Reviewed-on: https://go-review.googlesource.com/19721 Reviewed-by: Rob Pike <r@golang.org>