Add `len` method for `UnboundedSender` #2199

Job	Run time
cargo +1.36 build (futures-{core, io, sink})	1m 5s
cargo test (ubuntu-latest)	4m 26s
cargo test (macos-latest)	7m 30s
cargo test (windows-latest)	7m 35s
cargo test --target aarch64-unknown-linux-gnu	8m 33s
cargo test --target armv7-unknown-linux-gnueabihf	8m 35s
cargo test --target i686-unknown-linux-gnu	5m 34s
cargo +stable build	1m 18s
cargo +beta build	1m 4s
cargo +nightly build	1m 16s
cargo build --target thumbv7m-none-eabi	49s
cargo build --target thumbv6m-none-eabi	48s
cargo minimal-versions build	36s
cargo +1.56 build	1m 50s
cargo bench	1m 15s
cargo clippy	33s
codegen	33s
cargo doc	27s
cargo hack check --feature-powerset	4m 33s
cargo fmt	9s
cargo miri test	8m 43s
cargo test -Z sanitizer=address	4m 52s
cargo test -Z sanitizer=memory	3m 14s
cargo test -Z sanitizer=thread	3m 5s
	1h 18m 23s

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