[code-reflection] RFR: Make Quotable a marker interface [v10]

[Mourad Abbay]

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

Mourad Abbay has updated the pull request incrementally with three additional commits since the last revision:

 - Test that Op#ofQuotable returns same object for Quotable created via interpreter
 - Return an empty optional if Quotable is a proxy and add a note about this
 - Change the name of the magic method we call to get what's quoted

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Changes:
  - all: https://git.openjdk.org/babylon/pull/302/files
  - new: https://git.openjdk.org/babylon/pull/302/files/9da5b651..1613b2ab

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=302&range=09
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=302&range=08-09

  Stats: 30 lines in 4 files changed: 25 ins; 0 del; 5 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