encoding/json: unify encodeState.string and encodeState.stringBytes

encoding/json: unify encodeState.string and encodeState.stringBytes

This is part of the effort to reduce direct reliance on bytes.Buffer
so that we can use a buffer with better pooling characteristics.

Unify these two methods as a single version that uses generics
to reduce duplicated logic. Unfortunately, we lack a generic
version of utf8.DecodeRune (see #56948), so we cast []byte to string.
The []byte variant is slightly slower for multi-byte unicode since
casting results in a stack-allocated copy operation.
Fortunately, this code path is used only for TextMarshalers.
We can also delete TestStringBytes, which exists to ensure
that the two duplicate implementations remain in sync.

Performance:

    name              old time/op    new time/op    delta
    CodeEncoder          399µs ± 2%     409µs ± 2%   +2.59%  (p=0.000 n=9+9)
    CodeEncoderError     450µs ± 1%     451µs ± 2%     ~     (p=0.684 n=10+10)
    CodeMarshal          553µs ± 2%     562µs ± 3%     ~     (p=0.075 n=10+10)
    CodeMarshalError     733µs ± 3%     737µs ± 2%     ~     (p=0.400 n=9+10)
    EncodeMarshaler     24.9ns ±12%    24.1ns ±13%     ~     (p=0.190 n=10+10)
    EncoderEncode       12.3ns ± 3%    14.7ns ±20%     ~     (p=0.315 n=8+10)

    name              old speed      new speed      delta
    CodeEncoder       4.87GB/s ± 2%  4.74GB/s ± 2%   -2.53%  (p=0.000 n=9+9)
    CodeEncoderError  4.31GB/s ± 1%  4.30GB/s ± 2%     ~     (p=0.684 n=10+10)
    CodeMarshal       3.51GB/s ± 2%  3.46GB/s ± 3%     ~     (p=0.075 n=10+10)
    CodeMarshalError  2.65GB/s ± 3%  2.63GB/s ± 2%     ~     (p=0.400 n=9+10)

    name              old alloc/op   new alloc/op   delta
    CodeEncoder          327B ±347%     447B ±232%  +36.93%  (p=0.034 n=9+10)
    CodeEncoderError      142B ± 1%      143B ± 0%     ~     (p=1.000 n=8+7)
    CodeMarshal         1.96MB ± 2%    1.96MB ± 2%     ~     (p=0.468 n=10+10)
    CodeMarshalError    2.04MB ± 3%    2.03MB ± 1%     ~     (p=0.971 n=10+10)
    EncodeMarshaler      4.00B ± 0%     4.00B ± 0%     ~     (all equal)
    EncoderEncode        0.00B          0.00B          ~     (all equal)

    name              old allocs/op  new allocs/op  delta
    CodeEncoder           0.00           0.00          ~     (all equal)
    CodeEncoderError      4.00 ± 0%      4.00 ± 0%     ~     (all equal)
    CodeMarshal           1.00 ± 0%      1.00 ± 0%     ~     (all equal)
    CodeMarshalError      6.00 ± 0%      6.00 ± 0%     ~     (all equal)
    EncodeMarshaler       1.00 ± 0%      1.00 ± 0%     ~     (all equal)
    EncoderEncode         0.00           0.00          ~     (all equal)

There is a very slight performance degradation for CodeEncoder
due to an increase in allocation sizes. However, the number of allocations
did not change. This is likely due to remote effects of the growth rate
differences between bytes.Buffer and the builtin append function.
We shouldn't overly rely on the growth rate of bytes.Buffer anyways
since that is subject to possibly change in #51462.
As the benchtime increases, the alloc/op goes down indicating
that the amortized memory cost is fixed.

Updates #27735

Change-Id: Ie35e480e292fe082d7986e0a4d81212c1d4202b3
Reviewed-on: https://go-review.googlesource.com/c/go/+/469556
Run-TryBot: Joseph Tsai <joetsai@digital-static.net>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Auto-Submit: Joseph Tsai <joetsai@digital-static.net>