Move "extra" methods to extension traits (#306)

src/buf/buf_impl.rs

src/buf/buf_mut.rs

src/buf/ext/chain.rs

src/buf/ext/mod.rs

src/buf/ext/reader.rs

src/buf/ext/take.rs

src/buf/ext/writer.rs

src/buf/mod.rs

tests/test_chain.rs

tests/test_reader.rs

tests/test_take.rs