encoding/json: avoid allocation when decoding number types

encoding/json: avoid allocation when decoding number types

In CL 345488, we optimized strconv.ParseXXX for []byte arguments.
That allows immediate casting of a []byte to a string that does not escape to be copied on the stack.

Performance:

	goos: darwin
	goarch: arm64
	pkg: encoding/json
				old sec/op	new sec/op	delta
	CodeUnmarshal-10        3.019m ± 6%   2.865m ± 10%   -5.10% (p=0.043 n=10)
	CodeUnmarshalReuse-10   2.528m ± 4%   2.274m ± 13%  -10.03% (p=0.009 n=10)
	geomean                 2.762m        2.553m         -7.60%

				old B/s		new B/s		delta
	CodeUnmarshal-10        613.1Mi ± 5%   646.0Mi ±  9%   +5.37% (p=0.043 n=10)
	CodeUnmarshalReuse-10   732.1Mi ± 4%   813.7Mi ± 12%  +11.15% (p=0.009 n=10)
	geomean                 669.9Mi        725.0Mi         +8.22%

				old B/op	new B/op	delta
	CodeUnmarshal-10         2.782Mi ± 0%   1.918Mi ± 0%  -31.04% (p=0.000 n=10)
	CodeUnmarshalReuse-10   1600.8Ki ± 0%   713.3Ki ± 0%  -55.44% (p=0.000 n=10)
	geomean                  2.085Mi        1.156Mi       -44.57%

				old allocs/op 	new allocs/op 	delta
	CodeUnmarshal-10        91.31k ± 0%   39.99k ± 0%  -56.20% (p=0.000 n=10)
	CodeUnmarshalReuse-10   76.58k ± 0%   25.23k ± 0%  -67.06% (p=0.000 n=10)
	geomean                 83.62k        31.76k       -62.02%

Change-Id: I208c57089040daee0f9d979d1df725e3acf34f81
Reviewed-on: https://go-review.googlesource.com/c/go/+/518277
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Joseph Tsai <joetsai@digital-static.net>
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>