[foreign-memaccess+abi] RFR: 8264187: Add a method for creating a slicing MethodHandle

[Jorn Vernee]

Hi,

The PR https://github.com/openjdk/panama-foreign/pull/477 wen into the jextract branch by accident.

This is the same patch, but this time properly targeted at the memaccess+abi branch.

Thanks,
Jorn

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Commit messages:
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/MemoryLayout.java
 - Address review comments
 - Add some tests
 - add sliceHandle method

Changes: https://git.openjdk.java.net/panama-foreign/pull/479/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=479&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264187
  Stats: 147 lines in 3 files changed: 138 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/479.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/479/head:pull/479

PR: https://git.openjdk.java.net/panama-foreign/pull/479