[foreign-memaccess+abi] RFR: 8315769: Add support for sliced allocation [v4]

[Maurizio Cimadamore]

> This PR adds a new method in `SegmentAllocator`:
> 
> 
> default MemorySegment allocateFrom(ValueLayout elementLayout, MemorySegment source,
>                                        ValueLayout sourceElementLayout, long sourceOffset, long srcElementCount) {
> 
> 
> This method allows clients to allocate a new memory segment and copy the contents of a portion of an existing segment into the newly allocated region of memory. As such it can be used to address the following use cases:
> 
> * allocate from a `ByteBuffer`
> * allocate from another memory segment
> * allocate from a Java array slice
> 
> All these cases were not covered by the existing API points, which meant that developers had to use a more general allocation request (such as `allocate(long, long)`) and then pay a performance cost (because of memory zeroing). In other words, the new method in this PR completes the allocation API, by providing a flexible way to allocate a new segment from an existing source (another segment) with given offset and length.
> 
> Given that the new method is more general than the existing array-accepting `allocateFrom`, this PR rewires the existing array-accepting method to be rewritten on top of the new overload (and tweaks the javadoc of such methods accordingly).
> 
> One detail to note is that the new method takes _two_ element layouts - one is the layout of the newly allocated segment, whereas the other is the layout of the source segment. Such layouts must have same alignment and same carrier - but they can have different endianness (in which case a bulk copy with swap is performed). This is not too different from the most general `MemorySegment::copy` static overload.

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

  Use explicit layouts in array-accepting allocateFrom overloads

-------------

Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/878/files
  - new: https://git.openjdk.org/panama-foreign/pull/878/files/81e5c1ec..54fc1efa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=878&range=03
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=878&range=02-03

  Stats: 21 lines in 1 file changed: 0 ins; 7 del; 14 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/878.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign.git pull/878/head:pull/878

PR: https://git.openjdk.org/panama-foreign/pull/878