Floating point precision issues for Swift

[zongweil]

Hello from the main Open Location Code repo!

We're currently working on fixing floating point precision issues with the various implementations (see here). For the implementations not directly in the main repo, we're asking for the owners to update their code accordingly. Would you be able to do the following when you get a chance?

1. Run the new test cases in test_data/encoding.csv.
2. If they fail, fix your implementation to properly handle the floating point precision issues. I'll post again when we have a commit that you can use for reference.