buffer: add fast api for isUtf8 and isAscii

[gurgunday]

I like V8's fast api constraints, so I'm in the process of checking if there are any easy wins that we haven't yet spotted

Benchmarks (3 run avg, M5 Pro):

   Benchmark               Baseline avg       New avg      Delta
  ━━━━━━━━━━━━━━━━━━━━━━  ━━━━━━━━━━━━━━  ━━━━━━━━━━━━  ━━━━━━━━━
   isAscii short             35,991,851    41,781,518    +16.09%
  ──────────────────────  ──────────────  ────────────  ─────────
   isAscii long              36,288,837    41,568,699    +14.55%
  ──────────────────────  ──────────────  ────────────  ─────────
   isUtf8 regular short      62,235,173    77,679,543    +24.82%
  ──────────────────────  ──────────────  ────────────  ─────────
   isUtf8 unicode short      54,727,530    67,448,276    +23.24%
  ──────────────────────  ──────────────  ────────────  ─────────
   isUtf8 regular long       21,936,234    23,323,246     +6.32%
  ──────────────────────  ──────────────  ────────────  ─────────
   isUtf8 unicode long        1,415,756     1,424,730     +0.63%

main:

./node benchmark/run.js --filter buffer-isutf8 --filter buffer-isascii buffers
buffers/buffer-isascii.js
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 36,815,465.272773266
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 36,183,770.13174511
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 36,618,408.943863295
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 35,746,630.542265736
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 36,457,306.78481872
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 36,401,698.177805796

buffers/buffer-isutf8.js
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 62,364,681.698319875
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 55,971,859.319343686
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 22,072,747.830241162
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,428,668.0720477349
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 61,692,629.901998825
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 55,769,467.558882594
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 21,656,043.985478327
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,411,666.258768642
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 60,205,383.199316956
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 54,345,813.96273032
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 21,706,677.55977208
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,417,130.332308522

branch:

./node benchmark/run.js --filter buffer-isutf8 --filter buffer-isascii buffers
buffers/buffer-isascii.js
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 42,042,571.75319534
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 41,993,784.33620464
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 41,651,213.719283365
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 41,547,685.95250074
buffers/buffer-isascii.js input="hello world" length="short" n=20000000: 41,650,769.262218766
buffers/buffer-isascii.js input="hello world" length="long" n=20000000: 41,164,626.14506766

buffers/buffer-isutf8.js
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 78,129,501.6021431
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 67,884,812.56127538
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 23,347,893.348125998
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,438,952.7056895334
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 78,384,482.64411178
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 68,071,405.20227206
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 23,444,653.604879595
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,421,397.567430693
buffers/buffer-isutf8.js input="regular string" length="short" n=20000000: 76,524,646.0555287
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="short" n=20000000: 66,388,609.0424605
buffers/buffer-isutf8.js input="regular string" length="long" n=20000000: 23,177,192.30868906
buffers/buffer-isutf8.js input="∀x∈ℝ: ⌈x⌉ = −⌊−x⌋" length="long" n=20000000: 1,413,840.9062698097

Cc @nodejs/performance