Slice a MemorySegment using SequenceLayout's element boundaries

[Sebastian Stenzel]

Hi,

I've just updated some of my experiments [1] using the latest API in the upcoming JDK 18. Which lead to a question:

Given:

1. a MemoryAddress (parameter in an upcall stub) representing a pointer to an array of structs
2. a matching SequenceLayout of GroupLayouts for this array of structs

Is there any convenient API to obtain a MemorySegment of the nth array element? Currently I'm calculating memory boundaries manually and slice my segment accordingly:

```
private void upcall(MemoryAddress pointer) {
   try (var scope = ResourceScope.newConfinedScope()) {
      var structLayout = ...;
      var sequenceLayout = MemoryLayout.sequenceLayout(42, structLayout);
      var segment = MemorySegment.ofAddress(pointer, sequenceLayout.byteSize(), scope);

      // either:
      var offset0 = sequenceLayout.byteOffset(MemoryLayout.PathElement.sequenceElement(0));
      var offset1 = sequenceLayout.byteOffset(MemoryLayout.PathElement.sequenceElement(1));
      var elem0 = segment.asSlice(offset0, structLayout.byteSize());
      var elem1 = segment.asSlice(offset1, structLayout.byteSize());

      // or alternatively:
      var elem0 = segment.asSlice(0, structLayout.byteSize());
      var elem1 = segment.asSlice(structLayout.byteSize(), structLayout.byteSize());
   }
}
```

But since a SequenceLayout knows how to calculate the offset and knows the size of each of its elements, I guess there is a more convenient way to slice a segment.

Or if there isn't: Consider this a feature request ��

Cheers!
Sebastian

[1]: https://github.com/skymatic/fuse-panama/blob/develop/mac/src/main/java/de/skymatic/fusepanama/mac/MacFuseOperationsMapper.java#L178-L187