Functions that may panic can be annotated with #[track_caller] so that
in the event of a panic, the function where the user called the
panicking function is shown instead of the file and line within Tokio
source.

This change adds #[track_caller] to all the non-unstable public io APIs
in the main tokio crate where the documentation describes how the
function may panic due to incorrect context or inputs.

Additionally, the documentation for `AsyncFd` was updated to indicate
that the functions `new` and `with_intent` can panic.

Tests are included to cover each potentially panicking function. The
logic to test the location of a panic (which is a little complex), has
been moved to a test support module.

Refs: tokio-rs#4413

RawFd and AsRawFd aren't available on all systems, so the helper struct
for the AsyncFd tests need to be conditionally compiled for unix only.

Also fixed clippy warnings on windows where certain imports weren't
being used (because they are only used in tests with the unix cfg).