Add `len` method for `UnboundedSender` #2198

Job	Run time
cargo bench	1m 52s
cargo clippy	40s
cargo +stable build	58s
cargo +beta build	1m 4s
cargo +nightly build	1m 18s
codegen	37s
cargo +1.36 build (futures-{core, io, sink})	1m 5s
cargo test --target aarch64-unknown-linux-gnu	4m 38s
cargo test --target armv7-unknown-linux-gnueabihf	4m 36s
cargo test --target i686-unknown-linux-gnu	4m 38s
cargo doc	28s
cargo hack check --feature-powerset	4m 15s
cargo fmt	5s
cargo miri test	4m 34s
cargo minimal-versions build	40s
cargo build --target thumbv7m-none-eabi	55s
cargo build --target thumbv6m-none-eabi	46s
cargo test -Z sanitizer=address	4m 18s
cargo test -Z sanitizer=memory	4m 20s
cargo test -Z sanitizer=thread	3m 1s
cargo test (ubuntu-latest)	4m 28s
cargo test (macos-latest)	4m 30s
cargo test (windows-latest)	4m 40s
cargo +1.56 build	1m 27s
	59m 53s

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