buffer: optimize asciiWrite for short strings

Based on benchmark:

import { bench, group, run } from 'mitata'

const BUF = Buffer.allocUnsafe(64).fill(88)

function asciiWrite(str) {
  BUF.asciiWrite(str, 0)
}

function bufWrite(buf) {
  BUF.set(buf)
}

function loopWrite(src) {
  for (let n = 0; n < src.length; n++) {
    BUF[n] = src.charCodeAt(n)
  }
}

const str8 = '01234567'
const buf8 = Buffer.from(str8)
const str16 = '0123456789abcdef'
const buf16 = Buffer.from(str16)
const str32 = '0123456789abcdef'.repeat(2)
const buf32 = Buffer.from(str32)
const str64 = '0123456789abcdef'.repeat(4)
const buf64 = Buffer.from(str64)

group('8', () => {
  bench('asciiWrite', () => asciiWrite(str8))
  bench('loopWrite', () => loopWrite(str8))
  bench('bufWrite', () => bufWrite(buf8))
})
group('16', () => {
  bench('asciiWrite', () => asciiWrite(str16))
  bench('loopWrite', () => loopWrite(str16))
  bench('bufWrite', () => bufWrite(buf16))
})
group('32', () => {
  bench('asciiWrite', () => asciiWrite(str32))
  bench('loopWrite', () => loopWrite(str32))
  bench('bufWrite', () => bufWrite(buf32))
})
group('64', () => {
  bench('asciiWrite', () => asciiWrite(str64))
  bench('loopWrite', () => loopWrite(str64))
  bench('bufWrite', () => bufWrite(buf64))
})

await run()

cpu: Apple M2 Pro
runtime: node v22.2.0 (arm64-darwin)

benchmark       time (avg)             (min … max)       p75       p99      p999
-------------------------------------------------- -----------------------------
• 8
-------------------------------------------------- -----------------------------
asciiWrite   41.99 ns/iter     (39.88 ns … 247 ns)  41.65 ns  79.55 ns    182 ns
loopWrite     7.12 ns/iter    (6.94 ns … 70.11 ns)   7.04 ns   8.99 ns  21.55 ns
bufWrite      9.87 ns/iter      (9.56 ns … 219 ns)   9.75 ns  13.67 ns   18.8 ns

summary for 8
  loopWrite
   1.39x faster than bufWrite
   5.9x faster than asciiWrite

• 16
-------------------------------------------------- -----------------------------
asciiWrite   41.33 ns/iter     (39.86 ns … 333 ns)  41.32 ns  51.39 ns    149 ns
loopWrite    12.99 ns/iter     (12.49 ns … 125 ns)   12.7 ns     20 ns  45.02 ns
bufWrite     11.39 ns/iter    (9.97 ns … 3'374 ns)  10.23 ns  16.44 ns    101 ns

summary for 16
  bufWrite
   1.14x faster than loopWrite
   3.63x faster than asciiWrite

• 32
-------------------------------------------------- -----------------------------
asciiWrite   40.84 ns/iter     (39.86 ns … 164 ns)     41 ns  47.79 ns  61.28 ns
loopWrite    25.89 ns/iter   (24.25 ns … 1'722 ns)   24.8 ns  47.24 ns    256 ns
bufWrite     10.95 ns/iter   (10.74 ns … 75.05 ns)  10.86 ns  13.27 ns  19.47 ns

summary for 32
  bufWrite
   2.36x faster than loopWrite
   3.73x faster than asciiWrite

• 64
-------------------------------------------------- -----------------------------
asciiWrite   41.74 ns/iter     (40.77 ns … 244 ns)  41.91 ns  47.89 ns     85 ns
loopWrite    60.41 ns/iter   (54.73 ns … 3'905 ns)  56.52 ns  97.35 ns  2'027 ns
bufWrite      9.81 ns/iter      (9.56 ns … 375 ns)   9.66 ns     13 ns  19.57 ns

summary for 64
  bufWrite
   4.25x faster than asciiWrite
   6.16x faster than loopWrite