[code-reflection] RFR: Enhance OpWriter

[Paul Sandoz]

Enhance OpWriter to support the following options
1. Drop an operation's descendants. This allows for writing just the operation "header".
2. Write an operation's result even if it is void. This allows for improved debugging and presentation in some cases, esp. when we refer to values uniformly (an operation whose result is void still has a value, just one that is not used.)

Also, consolidate the write operations, and remove the explicit insertion of a live feed at the end of writing an operation.

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Commit messages:
 - Further simplify
 - Merge remote-tracking branch 'upstream/code-reflection' into op-writer-options
 - Simplify.
 - Update writer.

Changes: https://git.openjdk.org/babylon/pull/79/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=79&range=00
  Stats: 128 lines in 3 files changed: 77 ins; 42 del; 9 mod
  Patch: https://git.openjdk.org/babylon/pull/79.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/79/head:pull/79

PR: https://git.openjdk.org/babylon/pull/79