[code-reflection] RFR: Make Quotable a marker interface

[Mourad Abbay]

In this PR, we make Quotable a marker interface. This PR is based on #301.

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Commit messages:
 - Make Quotable a marker interface
 - Replace opField with opMethod for lambdas.

Changes: https://git.openjdk.org/babylon/pull/302/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=302&range=00
  Stats: 154 lines in 19 files changed: 50 ins; 26 del; 78 mod
  Patch: https://git.openjdk.org/babylon/pull/302.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/302/head:pull/302

PR: https://git.openjdk.org/babylon/pull/302