[foreign-memaccess] RFR: 8253355: Add some overloads to memory access API

[Maurizio Cimadamore]

This patch adds two overload:

* MemoryAddress::asSegmentRestricted which takes an Object attachment
* MemorySegment::asSlice which takes a MemoryAddress base and a new length

These overloads make the API slightly more usable.

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Commit messages:
 - Add overloads for MemorySegment::asSlice and MemoryAddress::asSegmentRestricted

Changes: https://git.openjdk.java.net/panama-foreign/pull/341/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=341&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253355
  Stats: 96 lines in 2 files changed: 88 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/341.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/341/head:pull/341

PR: https://git.openjdk.java.net/panama-foreign/pull/341