RFR: Fix jextract/panama to reflect latest API changes [v2]

[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`

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

  Fix samples

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Changes:
  - all: https://git.openjdk.org/jextract/pull/72/files
  - new: https://git.openjdk.org/jextract/pull/72/files/a53540cd..910ab267

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=72&range=01
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=72&range=00-01

  Stats: 5 lines in 2 files changed: 0 ins; 0 del; 5 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