RFR: Fix jextract/panama to reflect latest API changes [v3]

[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 whitespace

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Changes:
  - all: https://git.openjdk.org/jextract/pull/72/files
  - new: https://git.openjdk.org/jextract/pull/72/files/910ab267..c0a6c8c6

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jextract&pr=72&range=02
 - incr: https://webrevs.openjdk.org/?repo=jextract&pr=72&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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