[code-reflection] RFR: Enhance grammar of op attributes

[Maurizio Cimadamore]

This PR enhances the grammar of op attributes, so as to avoid use of "stringy" attributes where possible. Now all attributes that are Java primitives are externalized and re-parsed back automatically. This means that op factories no longer need to test for attribute values of type `String` and then manually parse that string back into the correct primitive, which makes the code more robust.

This change caused some reshuffling in the tests -- most notably, when fixing the triton test, the new logic highlighted what was probably an "issue" in the triton transformer (see related comment).

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Commit messages:
 - Update attribute grammar doc in OpParser
 - Simplify code in ArithMathOps.ConstantOp
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/435/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=435&range=00
  Stats: 960 lines in 52 files changed: 41 ins; 79 del; 840 mod
  Patch: https://git.openjdk.org/babylon/pull/435.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/435/head:pull/435

PR: https://git.openjdk.org/babylon/pull/435