tls_codec: use write_all instead of write

[keks]

I ran into an issue when writing into a base64 encoder. Debugging this turned up that the base64 encoder does not always write the entire provided buffer at once. In order to write into this cleanly, I just use write_all instead of write, and track the number of written bytes manually (i.e. not by inspecting the result of write - we now know that the whole slice was written).

I also added a test writer struct that only writes a single byte with each invocation of write_all, and a test that uses that for ensuring this is now handled correctly.

[franziskuskiefer]

Thanks! lgtm with a small nit.

The InvalidWriteLength is unused now I think. Let's add a comment to that effect (to avoid breakage we can keep it).

[franziskuskiefer]

Another nit in the test, then we can merge this.

[franziskuskiefer]

We should actually test something.
This is what's happening.

Suggested change

	first.tls_serialize(&mut short_writer).unwrap();
	assert_eq!(short_writer.0.len(), 35);
	assert_eq!(&short_writer.0[short_writer.0.len() - 3..], &[0, 1, 2]);