Implement tests (in abra!) for abra

[kengorab]

Now that there's the test module, it's time to write tests for Abra using Abra itself. It'll be helpful to have these be implemented in Abra rather than in Rust because if ever I get around to self-hosting (or building an alternate compilation target like in #300), I'll want these runnable using just Abra.

These are the areas that need to be covered

• Int
• Float
• Bool
• String
• Array
• Set
• Map
• Arithmetic & boolean operations
• Types/Enums
• Functions