Implement serial::{Tx,Rx}::{listen,unlisten}.

[jmgao]

This PR adds listen and unlisten to Tx and Rx, acting identically to Serial::listen/unlisten(Txe/Rxne).

(I'm not super happy with the duplication here, but it didn't seem trivial to extract it to a helper. Let me know if you want me to do so.)

[jmgao]

(build failure seems bogus)

$ rustup target add thumbv7m-none-eabi
info: downloading component 'rust-std' for 'thumbv7m-none-eabi'
error: component download failed for rust-std-thumbv7m-none-eabi
info: caused by: could not download file from 'https://static.rust-lang.org/dist/2019-05-23/rust-std-1.35.0-thumbv7m-none-eabi.tar.xz' to '/home/travis/.rustup/downloads/efbf39a10565886cab20a6278dd51702717ae4ca8de27cac7b937fff5d3f4ed7.partial'
info: caused by: failed to make network request
info: caused by: https://static.rust-lang.org/dist/2019-05-23/rust-std-1.35.0-thumbv7m-none-eabi.tar.xz: timed out

[TheZoq2]

Thanks for the PR, the failure does seems to be caused by travis.

That amount of duplication is no problem in my opinion, it's pretty much unavoidable.

Would you mind adding a changelog entry for this?

[jmgao]

Done.

[TheZoq2]

Perfect, thanks!