perf: make _occ Uint8Array

[gurgunday]

needle is a buffer, and _occ only takes buffer values (0-255), so it's more optimized (memory wise) to use an immutable Uint8Array here

Bench (node 22):

pr:

> busboy-benchmarks@1.0.0 benchmark-fastify
> node busboy/executioner.js -c 1

Mean time for fastify-busboy is 165742.53108811888 nanoseconds
all done

master:

> busboy-benchmarks@1.0.0 benchmark-fastify
> node busboy/executioner.js -c 1

Mean time for fastify-busboy is 166201.7394516819 nanoseconds
all done