RFR: universal type variables: initial prototype [v4]

[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:

  review comments, another iteration

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

Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/521/files
  - new: https://git.openjdk.java.net/valhalla/pull/521/files/3b300830..fe1a449a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=521&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=521&range=02-03

  Stats: 48 lines in 4 files changed: 10 ins; 22 del; 16 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