It can be useful to convert a MemoryAddress obtained from a native function call to a MemorySegment, so that fields can be read and written to.

We can help users doing this by adding a factory method such as:

    MemorySegment asSegmentRestricted(MemoryAddress addr) {...}

To the struct classes generated by jextract, and the implementation can use the MemoryLayout associated with the struct to do the conversion.

As a side note: currently RuntimeHelper has:

    MemorySegment asArrayRestricted(MemoryAddress addr, MemoryLayout layout, int numElements) {...}

To do something similar for arrays. This method could be moved to the CSupport class as well, since it is generally useful.