Update Bytes to Rust 2018 (#274)

Cargo.toml

README.md

benches/buf.rs

benches/bytes.rs

src/buf/buf.rs

src/buf/buf_mut.rs

src/buf/chain.rs

src/buf/from_buf.rs

src/buf/into_buf.rs

src/buf/iter.rs

src/buf/reader.rs

src/buf/take.rs

src/buf/writer.rs

src/bytes.rs

src/debug.rs

src/either.rs

src/lib.rs

src/serde.rs

tests/test_buf.rs

tests/test_buf_mut.rs