[foreign-memaccess+abi] RFR: 8301360: Add dereference layout paths to memory layout API

[Maurizio Cimadamore]

Now that we have a more general way to attach target layouts to address layouts (see https://git.openjdk.org/panama-foreign/pull/775), it might be useful to add support for layout paths which perform dereference of a given address value. Consider this case:


struct b {
   int x;
};

struct a {
    struct b *b;
};


Here, our goal is to construct a layout path for "a->b.x".

To do this, we use a *dereference path element* (see `PathElement#dereferenceElement`). These comes in two flavors:

* no argument factory - this assumes that the address layout being dereferenced has some target layout set;
* layout argument factory - this assumes that the address layout being dereferenced does *not* have a target layout set (e.g. it is not always possible to associate a target layout, especially in cyclic cases like a -> b -> a).

When a dereference element is applied to an existing path, a var handle that fetches the address pointed to by the current path is pushed on the stack, and a new layout path is started. This way, dereference path can be used to "chain" together multiple layout paths. All the intermediate memory read operations occur using a *plain* get. For more complex access operation (e.g. if atomic access is required for the whole dereference chain), multiple var handles have to be obtained.

It is an error to pass a dereference path to other layout operations such as `select`, `byte/bitOffset[Handle]`.

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

Commit messages:
 - Improve javadoc
 - Initial push

Changes: https://git.openjdk.org/panama-foreign/pull/776/files
 Webrev: https://webrevs.openjdk.org/?repo=panama-foreign&pr=776&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301360
  Stats: 312 lines in 3 files changed: 299 ins; 0 del; 13 mod
  Patch: https://git.openjdk.org/panama-foreign/pull/776.diff
  Fetch: git fetch https://git.openjdk.org/panama-foreign pull/776/head:pull/776

PR: https://git.openjdk.org/panama-foreign/pull/776