Add new Datatypes: Structs, Enums and Unions

[lucalewin]

Features
Add support for struct, enum and union datatypes

Solutions
The syntax of these datatypes should be something like this:

struct Test {
    var number: i64;
    var text: string;
    var character: char;
}

enum Test {
    ENUM_VAR_1,
    ENUM_VAR_2,
    ENUM_VAR_3;
}

The syntax of Unions should be very similar to Structs but they should behave differently

union Test {
    var number: i64;
    var text: string;
    var character: char;
}

Unions and Structs should work like the Structs and Unions in C whereas Enums should work differently.
The variables of an enum should not be globally accessible but accessible with an access expression like Test.ENUM_VAR_1

Additional context
The first version of this could just allocate the memory on the stack.