[foreign-jextract] RFR: 8246611: jextract generated allocate methods should accept CScope and callback allocate should auto-register

[Athijegannathan Sundararajan]

* Moved CScope as top-level class
* Cint, Cfloat,.. etc. use CScope instead of NativeAllocationScope
* Callback's have CScope accepting overload of 'allocate' method.
This allocate auto-registers callback segment with CScope.

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Commit messages:
 - 8246611: jextract generated allocate methods should accept CScope and callback allocate should auto-register

Changes: https://git.openjdk.java.net/panama-foreign/pull/196/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/196/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8246611
  Stats: 135 lines in 11 files changed: 68 ins; 51 del; 16 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/196.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/196/head:pull/196

PR: https://git.openjdk.java.net/panama-foreign/pull/196