Reduce allocations in poll(2) implementation

[Thomasdezeeuw]

This changes the poll(2) implementation to reuse to Vectors, so that we don't allocate in calls to Poll::poll in most cases. ¹ The downside to this is that Poll and Registry will contains two Vecs, which will increase their size. Especially for Registry this is annoying because they are never used. It might be worth considering exposing the same Poll/Registry split in the sys module to avoid this overhead for Registry.

This also contains a small cleanup commit to make the code match the style of the other files.

Closes #1701

/cc @jasta this is what I meant in #1687 (comment) and #1687 (comment)

Footnotes

1. Initially the vectors will be zero-sized and thus it will allocate in the first (couple of) calls, but at some point the capacity should be sufficient for further calls to no longer allocate. ↩

[Thomasdezeeuw]

Not doing this for now.