RFR: 7903344: refactor OutputFactory to introduce various Declaration Visitor classes

[Athijegannathan Sundararajan]

Separate visitors for each activity (duplicate removal, include symbol filtering, name mangling and output generation)

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Commit messages:
 - 7903344: refactor OutputFactory to introduce various Declaration Visitor classes

Changes: https://git.openjdk.org/jextract/pull/81/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=81&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903344
  Stats: 1409 lines in 18 files changed: 914 ins; 409 del; 86 mod
  Patch: https://git.openjdk.org/jextract/pull/81.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/81/head:pull/81

PR: https://git.openjdk.org/jextract/pull/81