Improve parsing performance

[natecook1000]

This converts some operations to constant time when parsing individual elements, leading to big performance improvements with larger argument lists.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test