[code-reflection] RFR: Replace opField with opMethod for lambdas.

[Mourad Abbay]

We replace field holding the textual representation of an identified lambda with method that returns the representation of that lambda.

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Commit messages:
 - Replace opField with opMethod for lambdas.

Changes: https://git.openjdk.org/babylon/pull/301/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=301&range=00
  Stats: 71 lines in 8 files changed: 13 ins; 16 del; 42 mod
  Patch: https://git.openjdk.org/babylon/pull/301.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/301/head:pull/301

PR: https://git.openjdk.org/babylon/pull/301