io: make read_to_end not grow unnecessarily

[tzx]

Fixes: #5594

Motivation

Explained in #5594

Solution

Make buf.reserve(32) do an additional check with our original capacity, so we can tell read_to_end to allocate a small buffer to read before reading into the main buffer. Doing so allows us to not allocate again when we reach capacity at an EOF, since we don't allocate with the check and read nothing into the small buffer. If we do not have reach an EOF however, we append out read data and continue as normal.

[tzx]

Alright I have moved logic outside of reserve and into its own function that poll_read_to_end uses. I also changed Vec::with_capacity to using a MaybeUninit array which uses unsafe.

[tzx]

Really wish I can catch these edge cases like you ahaha

[Darksonn]

Thanks, this looks good to me.