[foreign-memaccess+abi] RFR: 8301803: Rename MemorySegment::array

[Maurizio Cimadamore]

This patch picks up a loose end from Java 20: the name of `MemorySegment::array` is potentially too specific and will not scale when memory segment will be able to cover parts of Java objects contained flattened value types. The method is renamed here to `heapBase`, which is more general.

As I was fixing this, I realized that the behavior of this method was undocumented - that is, the method returns an empty optional for native segments. Also, I realized that for heap segments we did not check the reado-only status. The ByteBuffer does this, to prevent the contents of a read-only buffer to be written using the array. We should do the same here.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/809/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=809&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301803
  Stats: 19 lines in 6 files changed: 8 ins; 2 del; 9 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/809.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/809/head:pull/809

PR: https://git.openjdk.org/panama-foreign/pull/809