[foreign-jextract] RFR: Improve NIO channel support for buffer views over segments

[Chris Hegarty]

Initial prototype changes to use resource scope handles when performing I/O operations with synchronous and asynchronous channels.

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Commit messages:
 - remove explicit checkValidState check
 - Remove unnecessary abstract override
 - Prototype 2 Initial load

Changes: https://git.openjdk.java.net/panama-foreign/pull/512/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=512&range=00
  Stats: 1116 lines in 11 files changed: 1047 ins; 15 del; 54 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/512.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/512/head:pull/512

PR: https://git.openjdk.java.net/panama-foreign/pull/512