Enhance nested / ordered constant treatment

[jlowin]

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Please describe your work and make sure your PR:

• adds new tests (if appropriate)
• updates CHANGELOG.md (if appropriate)
• updates docstrings for any new functions or function arguments, including docs/outline.toml for API reference docs (if appropriate)

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What does this PR change?

This PR enhances the treatment of collections of constants, specifically nested collections and ordered collections.

Previously, collections of constants were represented as a Collection task with each item added as an upstream Constant (or Collection) dependency. This had two effects: first, it ignored order; second, it created a confusing graph because something like [1, 2, 3] would unexpectedly be represented by four tasks.

This PR examines nested objects to determine if they only contain constants. If they do, then the entire object is wrapped as a single Constant, preserving order and simplicity.

This PR also simplifies the convert_constants flag by removing it. Instead, if the result of as_task is a Constant, we simply add its value attribute to the flow's constants.

Why is this PR important?

As users are supplying more complex arguments to their tasks, they are discovering this undesirable behavior.

[codecov]

Codecov Report

Merging #1829 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

[cicdw]

Awesome

[jlowin]

@cicdw updated changelog for 0.8.0

[cicdw]

@jlowin sorry for the extremely delayed reviews on this - looks like there's a merge conflict in the changelog

[jlowin]

Updated!