RFR: universal type variables: initial prototype [v2]

[Vicente Romero]

> First iteration of the protype for universal type variables

Vicente Romero has updated the pull request incrementally with one additional commit since the last revision:

  addressing review comments

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/521/files
  - new: https://git.openjdk.java.net/valhalla/pull/521/files/e28bdb6a..39e77759

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=521&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=521&range=00-01

  Stats: 195 lines in 17 files changed: 19 ins; 109 del; 67 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/521.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/521/head:pull/521

PR: https://git.openjdk.java.net/valhalla/pull/521