[foreign-memaccess+abi] RFR: 8293400: Modify static constructors in MemorySegment [v2]

[Per Minborg]

> This PR changes the `MemorySegment` static constructors to be more like the one of `ByteBuffer`.

Per Minborg has updated the pull request incrementally with one additional commit since the last revision:

  Simplify creatons of MemorySegments in tests

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Changes:
  - all: https://git.openjdk.org/panama-foreign/pull/718/files
  - new: https://git.openjdk.org/panama-foreign/pull/718/files/da560367..4ebda409

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=panama-foreign&pr=718&range=01
 - incr: https://webrevs.openjdk.org/?repo=panama-foreign&pr=718&range=00-01

  Stats: 41 lines in 13 files changed: 0 ins; 0 del; 41 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/718.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/718/head:pull/718

PR: https://git.openjdk.org/panama-foreign/pull/718