Benchmark results

goos: linux
goarch: amd64
pkg: go.uber.org/zap/zapcore
cpu: AMD EPYC 7B13
               │ /tmp/old.txt │            /tmp/new.txt             │
               │    sec/op    │   sec/op     vs base                │
ZapJSON-8         89.10µ ± 1%   33.38µ ± 3%  -62.54% (p=0.000 n=10)
StandardJSON-8    40.74µ ± 1%   42.46µ ± 1%   +4.22% (p=0.000 n=10)
geomean           60.25µ        37.65µ       -37.52%

This no longer needs to be a separate function.

Adds a fuzz test for the string and []byte versions of safeAppendStringLike
that verifies that both variants are able to decode the original string
back.

The optimization is basically "instead of appending byte at a time,
skip over non-special bytes and append them all together."
The original optimization applies only to single-byte runes.
This applies the same to multi-byte runes.

Flips the logic a little to be easier to follow.
The shape is basically:

    if mutli byte rune {
        if no special handling {
            skip
            continue
        }
        special handling
    } else {
        if no special handling {
            skip
            continue
        }
        special handling
    }

This makes the logic much more obvious while retaining performance.