[foreign-memaccess] RFR: 8248487: Add static helpers to access segments [v2]

[Maurizio Cimadamore]

> This patch addresses some usability issues in the foreign memory API. First, it adds several static accessors for all
> carrier types in MemoryLayouts (I'm thorn whether to add this in MemoryLayouts or MemoryHandles, suggestions welcome).
> These accessors provide an access API that is similar to unsafe - all accessors expect a base address plus a byte
> offset. In most cases the accessors can be used and direct VH usage can be dropped, which might make approaching this
> API a bit easier.  Secondly, I've added a bunch of helpers to map a segment into _any_ array (as opposed to just
> `byte[]`). This makes the API more symmetric, given that, after all we _were_ able to create a segment from _any_ array
> (not just `byte[]`).

Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:

 - Add javadoc, and make the class final
 - Move accessors to separate class called MemoryAccess

-------------

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/221/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/221/files/2ca6bae3..5a77d956

Webrevs:
 - full: https://webrevs.openjdk.java.net/panama-foreign/221/webrev.01
 - incr: https://webrevs.openjdk.java.net/panama-foreign/221/webrev.00-01

  Stats: 1463 lines in 6 files changed: 748 ins; 697 del; 18 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/221.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/221/head:pull/221

PR: https://git.openjdk.java.net/panama-foreign/pull/221