[foreign-memaccess+abi] RFR: 8303666: Move address layout outside of ValueLayout namespace

[Maurizio Cimadamore]

This patch moves ValueLayout.OfAddress into a toplevel class. This seems desirable since address layouts are very important in the FFM API, and also provides seadditional methods on top of `ValueLayout` to get/set a *target layout*.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/811/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=811&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303666
  Stats: 245 lines in 31 files changed: 107 ins; 74 del; 64 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/811.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/811/head:pull/811

PR: https://git.openjdk.org/panama-foreign/pull/811