Time.appendFormatRFC3339 is a specialized implementation of
Time.appendFormat. We expect the two to be identical.
Add a fuzz test to ensure this property.

Updates #54093

Change-Id: I0bc41ee6e016d3dac75d1ac372d8c9c7266d0299
Reviewed-on: https://go-review.googlesource.com/c/go/+/425100
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>

RFC 3339 is the most common time representation,
being used in an overwhelming 57.3% of all specified formats,
while the next competitor only holds 7.5% usage.
Specially optimize parsing to handle the RFC 3339 format.
To reduce the complexity of error checking,
parseRFC3339 simply returns a bool indicating parsing success.
It leaves error handling to the general parse path.

To assist in fuzzing, the internal parse function was left unmodified
so that we could test that parseRFC3339 and parse agree with each other.

Performance:

	name             old time/op  new time/op  delta
	ParseRFC3339UTC  112ns ± 1%   37ns ± 1%    -67.37%  (p=0.000 n=9+9)
	ParseRFC3339TZ   259ns ± 2%   67ns ± 1%    -73.92%  (p=0.000 n=10+9)

Credit goes to Amarjeet Anand for a prior CL attemping to optimize this.
See CL 425014.

Fixes #54093

Change-Id: I14f4e8c52b092d44ceef6863f261842ed7e83f4c
Reviewed-on: https://go-review.googlesource.com/c/go/+/425197
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>