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

[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.

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

Commit messages:
 - Initial push

Changes: https://git.openjdk.java.net/panama-foreign/pull/480/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=480&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264386
  Stats: 122 lines in 11 files changed: 44 ins; 30 del; 48 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