fix: avoid unecessary concat calls

[zekth]

Because calling += operand is cost effective, reducing the number of calls seems like a good move to improve performances.

Reference benchmark

FJS creation x 36,866 ops/sec ±3.36% (74 runs sampled)
JSTR creation x 103,618 ops/sec ±3.28% (78 runs sampled)
CJS creation x 110,871 ops/sec ±2.26% (85 runs sampled)
JSON.stringify array x 3,308 ops/sec ±3.92% (79 runs sampled)
fast-json-stringify array x 5,923 ops/sec ±1.13% (86 runs sampled)
json-strify array x 48,848,583 ops/sec ±0.61% (91 runs sampled)
compile-json-stringify array x 6,132 ops/sec ±1.49% (85 runs sampled)
JSON.stringify long string x 13,234 ops/sec ±0.48% (93 runs sampled)
fast-json-stringify long string x 13,205 ops/sec ±0.44% (92 runs sampled)
json-strify long string x 250,959 ops/sec ±1.04% (91 runs sampled)
compile-json-stringify long string x 12,778 ops/sec ±0.97% (87 runs sampled)
JSON.stringify short string x 4,270,730 ops/sec ±1.23% (86 runs sampled)
fast-json-stringify short string x 24,189,271 ops/sec ±0.92% (93 runs sampled)
json-strify short string x 46,922,094 ops/sec ±0.60% (91 runs sampled)
compile-json-stringify short string x 27,792,567 ops/sec ±0.61% (95 runs sampled)
JSON.stringify obj x 1,998,108 ops/sec ±0.88% (88 runs sampled)
fast-json-stringify obj x 6,616,125 ops/sec ±1.11% (90 runs sampled)
json-strify obj x 50,441,958 ops/sec ±0.94% (93 runs sampled)
compile-json-stringify obj x 6,978,942 ops/sec ±1.06% (93 runs sampled)

Updated code benchmark

FJS creation x 45,313 ops/sec ±1.79% (89 runs sampled)
JSTR creation x 133,403 ops/sec ±0.98% (92 runs sampled)
CJS creation x 133,050 ops/sec ±1.25% (93 runs sampled)
JSON.stringify array x 4,338 ops/sec ±0.49% (93 runs sampled)
fast-json-stringify array x 6,842 ops/sec ±0.13% (98 runs sampled)
json-strify array x 55,356,698 ops/sec ±0.66% (95 runs sampled)
compile-json-stringify array x 6,841 ops/sec ±0.88% (91 runs sampled)
JSON.stringify long string x 14,075 ops/sec ±0.59% (94 runs sampled)
fast-json-stringify long string x 14,324 ops/sec ±0.42% (94 runs sampled)
json-strify long string x 277,193 ops/sec ±0.43% (94 runs sampled)
compile-json-stringify long string x 14,400 ops/sec ±0.36% (97 runs sampled)
JSON.stringify short string x 5,062,643 ops/sec ±0.40% (94 runs sampled)
fast-json-stringify short string x 26,170,935 ops/sec ±0.76% (96 runs sampled)
json-strify short string x 51,178,502 ops/sec ±0.47% (89 runs sampled)
compile-json-stringify short string x 29,463,421 ops/sec ±0.91% (95 runs sampled)
JSON.stringify obj x 2,208,298 ops/sec ±0.53% (96 runs sampled)
fast-json-stringify obj x 6,983,039 ops/sec ±0.41% (96 runs sampled)
json-strify obj x 53,769,162 ops/sec ±0.87% (94 runs sampled)
compile-json-stringify obj x 7,359,243 ops/sec ±0.27% (97 runs sampled)

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm

[mcollina]

lgtm, good spot!