RFR: 8266317: Vector API enhancements

[Paul Sandoz]

This PR contains API and implementation changes for [JEP-414 Vector API (Second Incubator)](https://openjdk.java.net/jeps/414), in preparation for when targeted.

Enhancements are made to the API for the support of operations on characters, such as for UTF-8 character decoding. Specifically, methods for loading/storing a `short` vector from/to a `char[]` array, and new vector comparison operators for unsigned comparisons with integral vectors. The x64 implementation is enhanced to supported unsigned comparisons.

Enhancements are made to the API for loading/storing a `byte` vector from/to a `boolean[]` array.

The testing of loads/stores can be expanded for scatter/gather, but before doing that i think some refactoring of the tests is required to reposition tests in the right classes. I would like to do that work after integration of the PR.

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Commit messages:
 - Minor clarifications to the specification.
 - 8266317: Vector API enhancements

Changes: https://git.openjdk.java.net/jdk/pull/3803/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3803&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266317
  Stats: 10002 lines in 121 files changed: 9070 ins; 190 del; 742 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3803.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3803/head:pull/3803

PR: https://git.openjdk.java.net/jdk/pull/3803