perf(nl): reuse itoa buffer for line numbering

[CrazyRoka]

Avoid re-allocating an itoa::Buffer for every line processed. By lifting the buffer creation out of the format_to function, we reduce memory pressure and improve performance when processing large files.

[xtqqczze]

The documentation for itoa::Buffer::new states: "This is a cheap operation; you don't need to worry about reusing buffers for efficiency."

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/pr/bounded-memory. tests/pr/bounded-memory is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tail/inotify-dir-recreate (fails in this run but passes in the 'main' branch)

[CrazyRoka]

Right, I see no improvement