RFR: 8318538: Add a way to obtain a strided var handle from a layout [v2]

[Maurizio Cimadamore]

> The 21 iteration of the FFM API used to contain a method, namely `ValueLayout::arrayElementVarHandle`. This var handle factory was useful when accessing elements in a flat array of variable size.
> 
> In 22, this method has been removed because:
> * It was not general enough (it only worked with value layouts)
> * A more useful/general version could be obtained with the following code:
> 
> 
> MethodHandles.collectCoordinates(varHandle(elements), 1, scaleHandle());
> 
> 
> Given that accessing variable-length array is rather common, and given that combining var handles is perceived as hard, we noticed that developers often tried to workaround by creating a sequence layout of maximal size, and then deriving a var handle from it. This feels suboptimal (as a new layout is created just so that we can select from it, and using this layout for anything else will likely lead to exceptions, because of its size).
> 
> For these reasons, we would like to expose this functionality as a new method in the `MemoryLayout` interface, namely `MemoryLayout::arrayElementVarHandle`. This method generalizes the old method in the 21 API, and is implemented in the obvious way, using a combinator (see above).
> 
> This PR also adds a lot of narrative text to describe how to deal with variable-length arrays, which should clarify the role of the new method.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16272/files
  - new: https://git.openjdk.org/jdk/pull/16272/files/31934027..ee6c3579

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16272&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16272&range=00-01

  Stats: 2 lines in 2 files changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/16272.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16272/head:pull/16272

PR: https://git.openjdk.org/jdk/pull/16272