[foreign-memaccess+abi] RFR: 8264933: Improve stream support in memory segments

[Maurizio Cimadamore]

This patch improves stream support for memory segments - it does so in two ways:

* first, tweaking the `MemorySegment::spliterator` method to take an *element layout*, instead of a full sequence layout whose size had to match that of the segment.
* secondly, by adding convenience methods to create streams directly, w/o needing to call `StreamSupport.stream`. As for `Collection`, two methods are provided: `MemorySegment::stream` and `MemorySegment::parallelStream`.

I've fixed up the documentation in places - I realized that, for example, the javadoc for `MemorySegment::spliterator` was missing a `@throws` clause for when there's a size mismatch between layout and segment.
Also, I've removed scope liveness check on the spliterator call - after all, a spliterator is going to make slices - a liveness check will occur when the slice is accessed.

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Commit messages:
 - Fix javadoc sample
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/494/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=494&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264933
  Stats: 77 lines in 5 files changed: 40 ins; 4 del; 33 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/494.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/494/head:pull/494

PR: https://git.openjdk.java.net/panama-foreign/pull/494