Minor Cleanup/Improvements

[ryantaylor]

This PR addresses various issues I've come across while using this library. A few are personal preference and will be labeled as such; others I believe are fundamental to the correct functioning of the library. If you'd like, I can separate the two and submit two PRs instead of this single one.

Changes

1. ok has been renamed to do_try. This has been done in the spirit of Rust's try macro, and makes more sense in my opinion. I believe ok should wrap a value in an :ok result in order to make chaining more user-friendly.
2. I've also added an err function to provide a user-friendly way to wrap a value in an :error result.
3. When using try_catch, a method that does not raise should return a value wrapped in an :ok result. This conforms to the expectations laid out in https://zohaib.me/railway-programming-pattern-in-elixir/ and makes sense logically, and also makes code much easier to write. Tests have been updated to reflect this change as well.
4. ExSpec has been updated to eliminate compiler warnings.

Of these changes, 3) and 4) are logical changes that I believe should be made no matter what; 1) and 2) are stylistic changes that I prefer but which I understand may not make sense to be included in a wide release. As mentioned, I can split the changes if that is preferred.