see
#2477 (review)

doing so saves me thinking on how to deal with pin and fields

- translate `std::io::LineWriter`'s unit test
- rename `inner` field to `buf_writer` to avoid confusion with
`BufWriter::inner`
- the above lead me to re-write the logic accordingly
- add methods to access `buf_writer`'s fields
- sorting out types, cannot compile for now

- default `poll_write_vectored` behaves differently of
`std::io::LineWriter::write_vectored`
- ported from
https://github.com/rust-lang/rust/blob/673d0db5e393e9c64897005b470bfeb6d5aec61b/library/std/src/io/buffered/tests.rs
- that code has lots of comments, you may wanna check it out
- translated tests for `std` 's `line_vectored`
- added `inner_poll_write_vectored` to keep code explicit and verbose