aya: perf_buffer: call BytesMut::reserve() internally

[alessandrod]

This changes PerfBuffer::read_events() to call BytesMut::reserve()
internally, and deprecates PerfBufferError::MoreSpaceNeeded.

This makes for a more ergonomic API, and allows for a more idiomatic
usage of BytesMut. For example consider:

let mut buffers = vec![BytesMut::with_capacity(N), ...];
loop {
    let events = oob_cpu_buf.read_events(&mut buffers).unwrap();
    for buf in &mut buffers[..events.read] {
        let sub: Bytes = buf.split_off(n).into();
        process_sub_buf(sub);
    }
    ...
}

This is a common way to process perf bufs, where a sub buffer is split
off from the original buffer and then processed. In the next iteration
of the loop when it's time to read again, two things can happen:

• if processing of the sub buffer is complete and sub has been
  dropped, read_events() will call buf.reserve(sample_size) and hit a fast
  path in BytesMut that will just restore the original capacity of the
  buffer (assuming sample_size <= N).

• if processing of the sub buffer hasn't ended (eg the buffer has been
  stored or is being processed in another thread),
  buf.reserve(sample_size) will actually allocate the new memory required
  to read the sample.

In other words, calling buf.reserve(sample_size) inside read_events()
simplifies doing zero-copy processing of buffers in many cases.