Add match statement

[Akuli]

Fixes #555

Example from docs:

enum Operation:
    Add
    Subtract
    Multiply
    Divide          # 7 / 3 produces 2.3333...
    FloorDivide     # 7 / 3 produces 2

def calculate(a: double, b: double, op: Operation) -> double:
    match op:
        case Operation.Add:
            return a + b
        case Operation.Subtract:
            return a - b
        case Operation.Multiply:
            return a * b
        case Operation.Divide:
            return a / b
        case Operation.FloorDivide:
            return floor(a / b)