Send runs artefacts into a dedicated `out` folder

Ensures that all raw data has at least an alignment of 128.

Empty `Vec`s have a dummy pointer that is set to the alignment of the item type. Thus, the pointer for empty `Vec<u8>` data is `0x1`. Using this pointer to construct a `ArrayData` instance can lead to alignment errors if the arrow array is of a type with larger alignment, e.g. float64.

This commit fixes the alignment error by checking for an empty raw buffer and constructing an empty `ArrayData` instance in this case.

Fixes #362

- If input has no data, return an empty `NullArray` instead of an empty byte array.
- If input contains Vec data (instead of shared memory data), use safe `arrow::buffer::Buffer::from_vec` constructor, which is also zero-copy.

Makes the distinction between raw data and data messages clearer.

- introduce a new `ArrayData` wrapper struct
- implement `TryFrom` to convert to primitive types and strings
- update examples to use the new conversion functions

The only difference betwenn the two traits is the additional Send bound. We should unify this again using the trait-level impl trait feature once it's stable.

Add convenience functions for merging external event streams with `EventStream`

Map shared memory region as read-only in receiver

Add convenience function `send_output_bytes`

Our upcoming ros2<->dora bridge will allow nodes to subscribe to ROS2 topics, which results in a second stream of events. By providing a merge function, this ROS2 event stream can be merged together with the dora event stream.

It's common to send a byte array that already exists, so it makes sense to provide a convenience function for this use case.

The maintainer of the `shared_memory` crate did not react to our PR, so I decided to create a (temporary) fork.

This ensures that the data is not modified, even if shared with Python as zero-copy arrow array. Modifying the data could result in undefined behavior since there might be other subscribers that read the data at the same time.