RFR: 8318678: Vector access on heap MemorySegments only works for byte[]

[Per Minborg]

This PR proposes to allow Vector access to any heap `MemorySegment` provided alignment restrictions are respected.

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Commit messages:
 - Allow Vector access to on heap MemorySegments of any type

Changes: https://git.openjdk.org/panama-vector/pull/218/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-vector&pr=218&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318678
  Stats: 269 lines in 41 files changed: 126 ins; 23 del; 120 mod
  Patch: https://git.openjdk.org/panama-vector/pull/218.diff
  Fetch: git fetch https://git.openjdk.org/panama-vector.git pull/218/head:pull/218

PR: https://git.openjdk.org/panama-vector/pull/218