Added future::on_poll, future::on_poll_pending and future::on_poll_ready #2568

Job	Run time
cargo test (ubuntu-latest)	3m 10s
cargo test (macos-latest)	6m 39s
cargo bench	45s
cargo test (windows-latest)	4m 39s
cargo +stable build	35s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 13s
cargo +1.36 build (futures-{core, io, sink})	27s
cargo +beta build	38s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 21s
cargo hack check --feature-powerset	2m 21s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 22s
cargo +nightly build	44s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 43s
cargo miri test	9m 39s
cargo build --target thumbv7m-none-eabi	28s
cargo +1.56 build	1m 0s
cargo build --target thumbv6m-none-eabi	28s
cargo clippy	25s
cargo doc	19s
cargo fmt	4s
cargo minimal-versions build	25s
cargo test -Z sanitizer=address	3m 29s
cargo test -Z sanitizer=memory	1m 14s
cargo test -Z sanitizer=thread	29s
	57m 37s

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