fix(core): Incorrect arg type on Fn.eachMemberIn

[RomainMuller]

Changed the type of the second argument to Fn.eachMemberIn to an array
of strings, such that it can be used at all.

BREAKING CHANGE: All instance methods of Fn were made static, and
the Fn constructor was made private.

Fixes #2950

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Pull Request Checklist

• Testing
  • Unit test added (prefer not to modify an existing test, otherwise, it's probably a breaking change)
  • CLI change?: coordinate update of integration tests with team
  • cdk-init template change?: coordinated update of integration tests with team
• Docs
  • jsdocs: All public APIs documented
  • README: README and/or documentation topic updated
  • Design: For significant features, design document added to design folder
• Title and Description
  • Change type: title prefixed with fix, feat and module name in parens, which will appear in changelog
  • Title: use lower-case and doesn't end with a period
  • Breaking?: last paragraph: "BREAKING CHANGE: <describe what changed + link for details>"
  • Issues: Indicate issues fixed via: "Fixes #xxx" or "Closes #xxx"
• Sensitive Modules (requires 2 PR approvers)
  • IAM Policy Document (in @aws-cdk/aws-iam)
  • EC2 Security Groups and ACLs (in @aws-cdk/aws-ec2)
  • Grant APIs (only if not based on official documentation with a reference)

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