[foreign-memaccess] RFR: Add MemorySegment::fill

[Chris Hegarty]

Hi,

As part of feedback on the Foreign Memory API (when experimenting with
its usage internally in the JDK), a small number of potential usability
enhancements could be made to the API. This is the second such.

This change proposes to add a new method:
  MemorySegment::fill

Which fills a value into the given memory segment. Fill can be useful to
initialize or reset the memory of a segment, similar(ish) to memset.

There are obviously different ways to provide such functionality, e.g.
accepting a fill value with a bit width greater than 8 bits, but on
balance this single method will likely satisfy the majority of simple
use-cases. Other more advanced initialization scenarios would likely be
served better from an interaction with the Vector API, or some such.

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

Commit messages:
 - minor fixes in fill
 - Merge branch 'foreign-memaccess' of https://github.com/openjdk/panama-foreign into fill
 - Add MemorySegment::fill

Changes: https://git.openjdk.java.net/panama-foreign/pull/161/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/161/webrev.00
  Stats: 137 lines in 3 files changed: 135 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/161.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/161/head:pull/161

PR: https://git.openjdk.java.net/panama-foreign/pull/161