[code-reflection] RFR: Immutble ExternalizedOp.attributes [v2]

[Paul Sandoz]

> The attributes map of `ExternalizedOp` no longer needs to be immutable. `OpBuilder` is modified accordingly to use `Map.of`.
> 
> In addition `OpBuilder` is also modified to reuse previously constructed type elements, and construct externalized ops using a constructor fewer parameters, if some are defaults.

Paul Sandoz has updated the pull request incrementally with one additional commit since the last revision:

  Share types.

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Changes:
  - all: https://git.openjdk.org/babylon/pull/407/files
  - new: https://git.openjdk.org/babylon/pull/407/files/c9e7addd..40650ec0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=babylon&pr=407&range=01
 - incr: https://webrevs.openjdk.org/?repo=babylon&pr=407&range=00-01

  Stats: 40 lines in 2 files changed: 28 ins; 0 del; 12 mod
  Patch: https://git.openjdk.org/babylon/pull/407.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/407/head:pull/407

PR: https://git.openjdk.org/babylon/pull/407