[foreign-abi] RFR: 8248499: Add methods to allocate off heap arrays from Java arrays [v3]

[Maurizio Cimadamore]

On Tue, 30 Jun 2020 12:41:15 GMT, Jorn Vernee <jvernee at openjdk.org> wrote:

>> Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Addressed review comments
>
> Marked as reviewed by jvernee (Committer).

I've addressed most of the comments; I also added an extra method `allocateArray(MemoryLayout, long)` to allocate an
'empty' array with given layout. This "just" a shortcut for `allocate(MemoryLayout.ofSequence(...))` but I think it's
very handy - e.g. to create empty pointer arrays, as in:

MemoryAddress addr = scope.allocateArray(C_POINTER, 42);

> Should this maybe be a public API in MemorySegment as well?

I'll prefer to leave that discussion for another patch

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PR: https://git.openjdk.java.net/panama-foreign/pull/222