RFR: 8318324: Drop redundant default methods from FFM API [v2]

[Maurizio Cimadamore]

> 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.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16224/files
  - new: https://git.openjdk.org/jdk/pull/16224/files/66123ead..9c3fe1d9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16224&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16224&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 6 del; 0 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