RFR: 7903799: apidiff: add initial code [v3]

[Jonathan Gibbons]

> Please review the initial code for the new apidiff tool.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  change `openjdk.java.net` to `openjdk.org`

-------------

Changes:
  - all: https://git.openjdk.org/apidiff/pull/2/files
  - new: https://git.openjdk.org/apidiff/pull/2/files/75924761..3cd9f85d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=apidiff&pr=2&range=02
 - incr: https://webrevs.openjdk.org/?repo=apidiff&pr=2&range=01-02

  Stats: 17 lines in 2 files changed: 0 ins; 0 del; 17 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