[foreign-memaccess+abi] RFR: 8264386: LibraryLookup should be more friendly with implicit unloading [v3]

[Maurizio Cimadamore]

> Quoting from the PR:
> 
> Dereferencing a segment from a LibraryLookup is possible, in the following form:
> 
> LibraryLookup libLookup = ...
> MemorySegment segment = libLookup.lookup("foo").asSegmentRestricted(....)
> 
> The above code has a problem: there is no link between the produced segment and the original library lookup - meaning that, in principle, it's possible for the library to be unloaded when dereference occurs.
> 
> After having discussed many alternatives with Jorn, the best way to address this conundrum would be to add an overload of `LibraryLookup::lookup` which takes a lyout and returns a segment.
> 
> As part of this change I have also removed the LibraryLookup.Symbol API which seems unnecessary - now you can lookup an address or a segment. Of course, looking up a segment is creating a restricted segment (so it should be treated as unsafe).
> 
> I've added a test to make sure that the new API works as expected.

Maurizio Cimadamore has updated the pull request incrementally with one additional commit since the last revision:

  Address review comments

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/480/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/480/files/399d30b3..5992d4d4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=480&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=480&range=01-02

  Stats: 3 lines in 2 files changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/480.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/480/head:pull/480

PR: https://git.openjdk.java.net/panama-foreign/pull/480