[code-reflection] RFR: Invokable block parameters

[Paul Sandoz]

The parameters of functions, lambdas, and closures (invokable operations) are modeled entry block parameters. This is a pattern or idiom of invokable operations. We add methods that help reason about that pattern.

We also a method to obtain the block's index in the parameter list of its declared block.

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Commit messages:
 - Update doc.
 - Test
 - Invokable block parameters

Changes: https://git.openjdk.org/babylon/pull/26/files
 Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=26&range=00
  Stats: 131 lines in 3 files changed: 128 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/babylon/pull/26.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/26/head:pull/26

PR: https://git.openjdk.org/babylon/pull/26