[foreign-jextract] RFR: API refresh - part two (jextract edition) [v3]

[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.

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

  Fix spurious reference to `stringAllocator`

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

Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/594/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/594/files/fff0fc6c..feb6bdc7

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=594&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=594&range=01-02

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