*: Replace Future impl with poll_next method

[elenaf9]

Fixes #22.

Remove Future implementation on all IfWatchers, instead add poll_next method to poll for address changed.
For the user-facing IfWatcher add Stream and FusedStream for convenience.

Probably easier to review the two commits separately.
466bd81 moves the poll logic from the Future impl into the new poll_next method and adds Stream + FusedStream.
9082c0e slightly refactors some of the poll_next implementation so that they are somewhat unified (following the conventions described in libp2p/rust-libp2p#2780):

• wrap the polling logic in a loop
• first return existing queued events
• poll for new events, continue in loop if progress was made

I think this makes it a bit easier to understand the different implementations, but I am fine with dropping that second commit in case that there are any concerns about the changes.

[mxinden]

Looks good to me apart from the version bump.

[mxinden]

👍

[mxinden]

@elenaf9 can you take a look at the CI failure?

error: unused import: `std::future::Future`
Error:   --> src/apple.rs:10:5
   |
10 | use std::future::Future;
   |     ^^^^^^^^^^^^^^^^^^^
   |
note: the lint level is defined here
  --> src/lib.rs:3:9
   |
3  | #![deny(warnings)]
   |         ^^^^^^^^
   = note: `#[deny(unused_imports)]` implied by `#[deny(warnings)]`

error: could not compile `if-watch` due to previous error