Cleaner cloner

[benjie]

Description

PgSelectStep and PgUnionAllStep had ugly constructors that were overloaded to allow for cloning versus creating new steps. Horrible.

This PR creates new static clone methods instead, and refactors the constructor to be simpler.

This technically isn't a breaking change because people should not be constructing classes directly, they should be using the helper functions as detailed in the docs. And even if they are constructing the classes directly they shouldn't be using that for cloning.

Performance impact

Marginal?

Security impact

If there's bugs in this, the bugs could be unpleasant.

Checklist

• My code matches the project's code style and yarn lint:fix passes.
• I've added tests for the new feature, and yarn test passes.
• I have detailed the new feature in the relevant documentation.
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists).
• If this is a breaking change I've explained why.

[changeset-bot]

🦋 Changeset detected

Latest commit: 5dc661e

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@dataplan/pg	Patch
graphile-build-pg	Patch
graphile-utils	Patch
pgl	Patch
postgraphile	Patch
graphile	Patch

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