reflect: add Value.SetZero

reflect: add Value.SetZero

The v.SetZero method is a faster equivalent of v.Set(Zero(v.Type())).

Performance:

	                     Direct         CachedZero     NewZero
	SetZero/Bool         2.20ns ± 0%    8.97ns ± 5%    11.4ns ± 1%
	SetZero/Int          3.08ns ± 1%    9.06ns ± 1%    11.5ns ± 0%
	SetZero/Uint         2.88ns ± 1%    9.04ns ± 1%    11.7ns ± 5%
	SetZero/Float        2.65ns ± 2%    9.05ns ± 1%    11.5ns ± 1%
	SetZero/Complex      2.68ns ± 3%    9.31ns ± 1%    11.7ns ± 1%
	SetZero/Array        6.69ns ± 4%    9.32ns ± 1%    11.8ns ± 1%
	SetZero/Chan         3.31ns ± 1%    6.19ns ± 1%    8.20ns ± 1%
	SetZero/Func         3.32ns ± 1%    6.20ns ± 0%    8.24ns ± 1%
	SetZero/Interface    2.66ns ± 1%    9.31ns ± 1%    11.8ns ± 1%
	SetZero/Map          3.31ns ± 1%    6.21ns ± 2%    8.19ns ± 1%
	SetZero/Pointer      3.30ns ± 1%    6.22ns ± 1%    8.17ns ± 1%
	SetZero/Slice        2.90ns ± 4%    9.13ns ± 1%    11.6ns ± 1%
	SetZero/String       3.11ns ± 1%    9.30ns ± 1%    11.8ns ± 2%
	SetZero/Struct       6.37ns ± 1%    9.18ns ± 4%    11.5ns ± 1%

where:

	* Direct is measuring Value.SetZero
	* CachedZero is measuring Value.Set with a cached Zero value
	* NewZero is measuring Value.Set with a new Zero value

Fixes #52376

Change-Id: I793ca723aa97627824c5f5b356b2da30c8e46d71
Reviewed-on: https://go-review.googlesource.com/c/go/+/411476
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Reviewed-by: Dan Kortschak <dan@kortschak.io>