RFR: Fix jextract/panama to reflect latest API changes

[Maurizio Cimadamore]

This patch tweaks the jextract implementation to take into account two API changes:

* `MemorySegment.allocateNative` no longer accepts a session parameter (in all cases we were passing an implicit session, so that's ok)
* `Linker.methodType` has been moved to `FunctionDescriptor::toMethodType`

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

Commit messages:
 - Initial push

Changes: https://git.openjdk.org/jextract/pull/72/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=72&range=00
  Stats: 9 lines in 5 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.org/jextract/pull/72.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/72/head:pull/72

PR: https://git.openjdk.org/jextract/pull/72