RFR: 8318324: Drop redundant default methods from FFM API

Wed Oct 18 12:10:25 UTC 2023

This PR moves turns default methods in `MemorySegment` and `MemoryLayout` into ordinary abstract methods.
These interfaces cannot be subclasses by developers (they are sealed), so there is no reson for having default methods there. Also, default methods can have some detrimental effects, as they cause the interfaces to be initialized more eagerly.

-------------

Commit messages:
 - Fix whitespaces
 - Drop redundant imports
 - Initial push

Changes: https://git.openjdk.org/jdk/pull/16224/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16224&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318324
  Stats: 628 lines in 4 files changed: 357 ins; 221 del; 50 mod
  Patch: https://git.openjdk.org/jdk/pull/16224.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16224/head:pull/16224

PR: https://git.openjdk.org/jdk/pull/16224