[foreign-jextract] RFR: 8244055: jextract should generate struct and union classes as nested classes of header file class

[Athijegannathan Sundararajan]

* generating nested classes for structs and unions
* using MemoryAddress instead of MemorySegment as first argument for struct/union getters/setters
* added indexed field setters and getters for array element set/get
* added tests for struct array creation, indexed access get/set

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Commit messages:
 - * Accepting MemoryAddress instead of MemorySegment for struct (array access)
 - * fixed alignment for nested struct classes
 - 8244055: jextract should generate struct and union classes as nested classes of header file class

Changes: https://git.openjdk.java.net/panama-foreign/pull/134/files
 Webrev: https://webrevs.openjdk.java.net/panama-foreign/134/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8244055
  Stats: 153 lines in 8 files changed: 122 ins; 3 del; 28 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/134.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/134/head:pull/134

PR: https://git.openjdk.java.net/panama-foreign/pull/134