[foreign-preview] RFR: 8282026: Remove support for unbound sequence layouts

[Maurizio Cimadamore]

When doing a pass over the memory layout API, I couldn't help noting that the API implementation and specification are made more complex by the fact that we have to specify what happens when a sequence layout *without a size* is found. Support for unbounded sequence layout was added many moons ago, with the understanding that it would have made life for jextract easier.

This is not the case; all cases where jextract is using an unbounded sequence layout today can be replaced with using a simple zero-length sequence (which is allowed by the API, as are empty structs/unions).

I also found that the only real use case for using unbounded sequence layout came from a quirk in the API to obtain var handles from layout. That is, if we want to obtain an indexed var handle into a JAVA_INT, we have to do the following:


VarHandle intHandle = MemoryLayout.newSequenceLayout(JAVA_INT)
            .varHandle(PathElement.sequenceLayout());


In hindsight, this is just boilerplate: in this case the user only cares about the element layout (`JAVA_INT`), but the API wants the user to wrap that element layout in a new sequence layout, only to then provide a path inside the sequence layout (which will add a free dimension to the resulting var handle).

It is not hard to see that all the above can be simplified, by adding an extra API method on memory layouts:


VarHandle intHandle = JAVA_INT.arrayElementVarHandle(JAVA_INT);


This more explicit, succint, and provdies less opportunities for bugs to hide.

Therefore, this patch removes support for unbounded sequence element; the net effect is that now _all_ layouts have a size - which significantly simplifies the use sites where layout sizes are computed (no need to use optional there). All without losing much in terms of expressiveness: zero-length sequence layout can be used in basically the same way in which unbounded sequence layouts were used before.

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Commit messages:
 - Improve javadoc for arrayXYZHandle
 - Revert spurious changes to TestLayouts
 - Fix LayoutPath
 - Tweak javadoc
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/642/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=642&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282026
  Stats: 315 lines in 36 files changed: 48 ins; 172 del; 95 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/642.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/642/head:pull/642

PR: https://git.openjdk.java.net/panama-foreign/pull/642