[foreign-jextract] RFR: Add MemoryLayout::byteOffset

[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 first such.

This change:
 1) renames MemoryLayout::offset to MemoryLayout::bitOffset, and
 2) adds MemoryLayout::byteOffset

This allows for easier interoperation with MemoryAddress, which deals
with offsets and lengths in terms of bytes (rather than bits), e.g.
   addr.addOffset(layout.byteOffset(groupElement("foo")))

This brings a nice symmetry to the API; there is a trio of bit-wise
methods: bitAlignment, bitOffset, bitSize, and their matching byte-wise
counterparts; byteAlignment, byteOffset, byteSize.

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

Commit messages:
 - fix typo in javadoc example
 - Initial MemoryLayout::byteOffset and test changes

Changes: https://git.openjdk.java.net/panama-foreign/pull/157/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/157/webrev.00
  Stats: 131 lines in 2 files changed: 103 ins; 1 del; 27 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/157.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/157/head:pull/157

PR: https://git.openjdk.java.net/panama-foreign/pull/157