Add more tests

[l2zou]

Currently, the compiler and interpreter are tested during integration tests. Due to the recent compiler rewrite, they should be moved inside compiler.rs and interpreter.rs, respectively.

Tests should also be added for lexer.rs and optimizer.rs once it is implemented.

[l2zou]

Compiler and interpreter tests have been moved.

Lexer tests have been added and optimizer tests will be added once it's implemented.

[l2zou]

This issue is in place again now that functions and blocks have been added.