[foreign-memaccess] RFR: 8252504: Add a method to MemoryLayout which returns a offset-computing method handle

[Jorn Vernee]

This PR adds two new methods: `bitOffsetHandle` and `byteOffsetHandle` to `MemoryLayout`, that can be used to create method handles that can compute the effective offset of a layout element given a set of additional `long` indexes.

I also cleaned up a leftover in the javadoc of bitOffset and byteOffset where an API note was stating that `sequenceElement()` layout path elements would be interpreted as `sequenceElement(0)`, but they were actually being rejected outright (as they should).

I also added some tests to cover the corner case where an UnsupportedOperationException was being thrown. (some of the churn in the tests is from introducing static imports. I hope it's not too noisy).

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Commit messages:
 - Add positive byteOffsetHandle test, and test that tests a bit offset that is not a multiple of 8
 - Polish impl
 - Remove unused imports
 - Added more tests, touched up javadoc
 - WIP - javadoc
 - WIP

Changes: https://git.openjdk.java.net/panama-foreign/pull/396/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=396&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252504
  Stats: 275 lines in 5 files changed: 216 ins; 7 del; 52 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/396.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/396/head:pull/396

PR: https://git.openjdk.java.net/panama-foreign/pull/396