RFR: universal type variables: initial prototype

[Vicente Romero]

First iteration of the protype for universal type variables

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Commit messages:
 - universal type variables: initial prototype

Changes: https://git.openjdk.java.net/valhalla/pull/521/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=521&range=00
  Stats: 854 lines in 24 files changed: 816 ins; 1 del; 37 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