strconv: format hex floats

This CL updates FormatFloat to format
standard hexadecimal floating-point constants,
using the 'x' and 'X' verbs.

See golang.org/design/19308-number-literals for background.

For #29008.

Change-Id: I540b8f71d492cfdb7c58af533d357a564591f28b
Reviewed-on: https://go-review.googlesource.com/c/160242


Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>