RFR: addressing several issues in the implementation of universal tvars [v6]

[Vicente Romero]

> Last review iteration found several issues in the current implementation, addressing them here

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

  adding structural type comparator to detect unchecked conversions

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Changes:
  - all: https://git.openjdk.java.net/valhalla/pull/684/files
  - new: https://git.openjdk.java.net/valhalla/pull/684/files/d005964b..6722ef71

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=valhalla&pr=684&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=valhalla&pr=684&range=04-05

  Stats: 466 lines in 5 files changed: 216 ins; 214 del; 36 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/684.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/684/head:pull/684

PR: https://git.openjdk.java.net/valhalla/pull/684