RFR: 8376040: Add prototype "textbook complex" class

[Joe Darcy]

First cut at a version of complex numbers using the textbook algorithms for the arithmetic operations. This version is intended for prototyping and _not_ for production use.

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Commit messages:
 - Appease jcheck.
 - 8376040: Add prototype "textbook complex" class

Changes: https://git.openjdk.org/valhalla/pull/1942/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1942&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8376040
  Stats: 446 lines in 1 file changed: 446 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/valhalla/pull/1942.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1942/head:pull/1942

PR: https://git.openjdk.org/valhalla/pull/1942