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

[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.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/878/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=878&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315769
  Stats: 159 lines in 2 files changed: 136 ins; 6 del; 17 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