hrw4u: native C++ parser and visitor for header_rewrite

[zwoop]

This essentially converted the existing Python parser / visitor / emitters to C++ and natively calling the various factory functions to create the object structure. In addition, it adds a new testing tool, hrw_confcmp, which compares an old vs a hrw4u configuration, and make sure they both create the same set of objects. As such, the header_rewrite core also added some support to identify the various Operators and Conditions. Usage:

leif@hel ~/BUILDS/ats-dbg » ./tools/hrw_confcmp/run_tests.sh  ~/apache/trafficserver/tools/hrw4u/tests/data/{hooks,conds,ops,vars,examples}

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Testing directory: hooks
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  ✓ hooks/read_response:             44 ms
  ✓ hooks/remap:                     39 ms
.
.
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  ✓ examples/uuid:                   41 ms
  ✓ examples/x-debug:                41 ms

  Directory Summary: 18 passed, 0 failed (total: 18)

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Overall Summary
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  Total tests:  66
  Passed:       66
  Failed:       0

  Total time:   2721 ms
  Avg per test: 41 ms

✓ All tests passed!

[zwoop]

I'm making this a draft for now, I think it needs to go into ATS v11, since we're getting close to 10.2 branching and release.