Reorder fields to reduce `sizeof(asio_event_t)` to 40 bytes (#4190)
Reorder fields to reduce `sizeof(asio_event_t)` to 40 bytes (#4190)

Reduces `sizeof(struct asio_event_t)` from 48 bytes to 40 bytes on
64-bit x86 Linux/FreeBSD. The `noisy` field was talking up 3 bytes due
to aligment of the following 64-bit value `nsecs` at a word boundary
(offset 32).  The two other flags `readable` and `writable` fit in the
alignment hole and no longer waste 8 bytes to align the whole struct at
word boundaries again.
jemc/ponyc,ponylang/ponyc,ponylang/ponyc,jemc/ponyc,jemc/ponyc,ponylang/ponyc