[foreign-jextract] RFR: API refresh - part two (jextract edition)

[Maurizio Cimadamore]

This patch fixes jextract to work with the latest API changes.

The patch reintroduces overloads for foreign function wrappers which take a scope (since ResourceScope is no longer a subtype of SegmentAllocator).

We will consider later whether to drop these overloads; it is likely that the majority of jextract clients will already have an allocator somewhere, so the extra code is not necessary. On the other hand, if we want overloads, then I think it would also be useful to add an overload that takes a `MemorySegment` and converts it into a `prefixAllocator` (so that clients can store result of foreign function which returns a struct-by-value in a pre-existing segments).

I've made some improvements to the string conversion logic; in a lot of places we were creating a scope, then doing a malloc to allocate a temporary CXString, then extract a Java String from the CXString, and dispose/free the CXString. Since jextract is single-threaded, I replaced all that with an high-order function which takes a segment supplier, and uses a prefix allocator under the hood. This should be way more efficient than what we had - and string conversion is quite frequent in libclang (e.g. `CXCursor::spelling`).

The patch passes all tests.

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

Commit messages:
 - Merge branch 'foreign-jextract' into cleanup_allocator+jextract
 - Use a prefix allocator for string conversions
 - Fix jextract to reflect latest API changes
 - Merge branch 'cleanup_allocator' into cleanup_allocator+jextract
 - Some more javadoc fixes on ResourceScope
 - Revert CallOverheadHelper to use shared/confined points
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/ResourceScope.java
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/SegmentAllocator.java
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/ResourceScope.java
 - Update src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/ResourceScope.java
 - ... and 22 more: https://git.openjdk.java.net/panama-foreign/compare/ea1e0d6e...8af282d7

Changes: https://git.openjdk.java.net/panama-foreign/pull/594/files
 Webrev: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=594&range=00
  Stats: 201 lines in 18 files changed: 99 ins; 26 del; 76 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/594.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/594/head:pull/594

PR: https://git.openjdk.java.net/panama-foreign/pull/594