RFR: 7903799: apidiff: add initial code

[Jonathan Gibbons]

Please review the initial code for the new apidiff tool.

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Commit messages:
 - fix whitespace
 - fix COPYRIGHT date
 - 7903799: apidiff: add initial code

Changes: https://git.openjdk.org/apidiff/pull/2/files
  Webrev: https://webrevs.openjdk.org/?repo=apidiff&pr=2&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903799
  Stats: 40655 lines in 175 files changed: 40655 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/apidiff/pull/2.diff
  Fetch: git fetch https://git.openjdk.org/apidiff.git pull/2/head:pull/2

PR: https://git.openjdk.org/apidiff/pull/2