[code-reflection] RFR: Lowering transformer

[Paul Sandoz]

An an implementation of a lowering transformer to `OpTransformer`, and replace explicitly lowering with that implementation.

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Commit messages:
 - Lowering transformer.

Changes: https://git.openjdk.org/babylon/pull/75/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=75&range=00
  Stats: 566 lines in 39 files changed: 46 ins; 454 del; 66 mod
  Patch: https://git.openjdk.org/babylon/pull/75.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/75/head:pull/75

PR: https://git.openjdk.org/babylon/pull/75