Rewrite formatFloat to be much simpler and clearer and
avoid the tricky interaction with padding.
The issue refers to complex but the problem is just floating-point.
The new tests added were incorrectly formatted before this fix.
Fixes #8064.

LGTM=jscrockett01, rsc
R=rsc, jscrockett01
CC=golang-codereviews
https://golang.org/cl/99420048