Update builtin macros to use unique IDs.

[jnthntatum]

Filter and map macros previously reused Expr.id values for repeated references to the accumulator var. After this change, the parser should always provide unique node IDs per AST assuming standard macros.

Pull Requests Guidelines

See CONTRIBUTING.md for more details about when to create
a GitHub Pull Request and when other kinds of contributions or
consultation might be more desirable.

When creating a new pull request, please fork the repo and work within a
development branch.

Commit Messages

• Most changes should be accompanied by tests.
• Commit messages should explain why the changes were made.

Summary of change in 50 characters or less

Background on why the change is being made with additional detail on
consequences of the changes elsewhere in the code or to the general
functionality of the library. Multiple paragraphs may be used, but
please keep lines to 72 characters or less.

Reviews

• Perform a self-review.
• Make sure the Travis CI build passes.
• Assign a reviewer once both the above have been completed.

Merging

• If a CEL maintaner approves the change, it may be merged by the author if
  they have write access. Otherwise, the change will be merged by a maintainer.
• Multiple commits should be squashed before merging.
• Please append the line closes #<issue-num>: description in the merge message,
  if applicable.