[foreign-memaccess] RFR: 8249790: Add Addressable abstraction

[Maurizio Cimadamore]

This patch adds a new entity in the Foreign Memory Access API, namely `Addressable`. This is a simple functional
interface which is implemented by all the entities which can be mapped onto a `MemoryAddress`. For now, only
`MemoryAddress` and `MemorySegment` implements it (but on the foreign-abi branch we will repeat the exercise and add
more implementations).

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Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/252/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/252/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8249790
  Stats: 234 lines in 30 files changed: 28 ins; 5 del; 201 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/252.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/252/head:pull/252

PR: https://git.openjdk.java.net/panama-foreign/pull/252