Implement more Python syntax

[almarklein]

• [x] Basic arithmetic #16
• [x] Casting #16
• [x] Tuple swapping (a, b = b, a)
• [ ] Setting vector attributes (color.a = 1.0)
• [x] In-place ops (a += 1)
• [x] Unary expressions (-a)
• [x] Ternary expressions
• [x] if-statements
• [x] loops
• [x] Calling SpirV builtin functions