Rust: Make Encoder rollback to previous buffer on error

[Vadmeme]

As it stands the Encoder api on error leaves junk in Encoder.buffer forcing user code to work around it

let buf = encoder.take_buffer();
let len = buf.len();
encoder.set_buffer(buf);

if let Ok(l) = encoder.encode(instruction, ip) {
    // ...
} else {
    let mut buf = encoder.take_buffer();
    buf.truncate(len);
    encoder.set_buffer(buf);
}

this PR makes the buffer truncate happen directly in the Encoder::encode call