Simplify split api

[Ddystopia]

Shortly, this PR merged split and split_with into a single function and removed unsplit.

let (mut reader, mut writer) = tls.split();

std::thread::scope(|scope| {
    scope.spawn(|| {
        let mut buffer = [0; 4];
        reader.read_exact(&mut buffer).expect("Failed to read data");
    });
    scope.spawn(|| {
        writer.write(b"ping").expect("Failed to write data");
        writer.flush().expect("Failed to flush");
    });
});

tls.close()
    .map_err(|(_, e)| e)
    .expect("error closing session");

Instead of

let mut state = SplitConnectionState::default();
let (mut reader, mut writer) = tls.split_with(&mut state);

std::thread::scope(|scope| {
    scope.spawn(|| {
        let mut buffer = [0; 4];
        reader.read_exact(&mut buffer).expect("Failed to read data");
    });
    scope.spawn(|| {
        writer.write(b"ping").expect("Failed to write data");
        writer.flush().expect("Failed to flush");
    });
});

let tls = TlsConnection::unsplit(reader, writer);

tls.close()
    .map_err(|(_, e)| e)
    .expect("error closing session");

Splitting is a common pattern in Rust, but embedded-tls had implemented it in a peculiar way. Unfortunately, this way is not ergonomic and does not compose well.

Commonly, split exclusively borrows self and returns two sides which are using it's data. Original entity cannot be used because it is mutably borrow while children are alive. When both dropped, parent is automatically released by the borrow checker.

A good example is heapless::Queue::split

I tried to use embedded-tls with other, traditional, "split" APIs, but unsplit approach just doesn't work here due to variance issues - you need to have &mut to put back reader and writer, so you can use unsplit, but reader and writer are not static, thus lifetime becomes invariant, making it unusable. You can read more about variance in the corresponding rustonomicon section.

That PR converted split to the borrowing version and removed a number of not really needed types. Implementations of buffers became a little bit more explicit, to improve borrow checker's behavior.

[lulf]

@Ddystopia Thanks for the PR, I merged it in #152.