Add accessors for the inner of stream::Iter #2746

Job	Run time
cargo +1.56 build	1m 20s
cargo +1.36 build (futures-{core, io, sink})	59s
cargo +stable build	17s
cargo bench	19s
cargo hack check --feature-powerset	17s
cargo +beta build	24s
cargo +nightly build	17s
cargo minimal-versions build	17s
cargo miri test	37s
cargo build --target thumbv7m-none-eabi	21s
cargo build --target thumbv6m-none-eabi	20s
cargo test -Z sanitizer=address	36s
cargo test (ubuntu-latest)	17s
cargo test -Z sanitizer=memory	36s
cargo test (macos-latest)	25s
cargo clippy	13s
cargo test -Z sanitizer=thread	34s
cargo test (windows-latest)	1m 1s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	47s
cargo doc	15s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	38s
cargo fmt	10s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	45s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	33s
	12m 18s

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