RFR: 8376040: Add prototype "textbook complex" class [v2]

[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.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review comments, cleanup.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1942/files
  - new: https://git.openjdk.org/valhalla/pull/1942/files/03baac7f..028b54af

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1942&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1942&range=00-01

  Stats: 22 lines in 1 file changed: 1 ins; 13 del; 8 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