[foreign-memaccess] RFR: JDK-8242051: Clarify difference between checked and unchecked addresses

[Maurizio Cimadamore]

While most memory addresses are associated with a segment, addresses coming from native code, or those created from
scratch (e.g. MemoryAddress::ofLong) do not have an associated segment.

This patch addresses this problem by having unchecked memory addresses returning`null` on MemoryAddress::segment().
Similarly, the MemoryAddress::offset method has been renamed to `segmentOffset` and now throws UOE if called on an
unchecked address.

This makes the API cleaner: it is only possible to dereference those addresses which have an associated segment. I've
tweaked the javadoc to convey that message more clearly.

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Commit messages:
 - * Tweaked API to return null if segment attached to MemoryAddress is nothing segment

Changes: https://git.openjdk.java.net/panama-foreign/pull/90/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/90/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-JDK-8242051
  Stats: 50 lines in 6 files changed: 23 ins; 2 del; 25 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/90.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/90/head:pull/90

PR: https://git.openjdk.java.net/panama-foreign/pull/90