[lworld] RFR: 8231500: [lworld] Merge the experimental bytecode API

[David Simms]

* Moved to a single bytecode testing API
* Cleaned out vbytecodes
* Cleanup of tests not actually using the API
* Removed TestNativeClone's API usage since cooking the method handle could no
longer bypass the verifier's bad access check

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Commit messages:
 - Added defaultvalue & withfield to test API and realigned Valhalla test to the test API
 - Remove TYPED bytecode
 - Remove vbytecodes
 - Bytecode API in one place

Changes: https://git.openjdk.java.net/valhalla/pull/279/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=279&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8231500
  Stats: 9334 lines in 65 files changed: 280 ins; 8939 del; 115 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/279.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/279/head:pull/279

PR: https://git.openjdk.java.net/valhalla/pull/279