Structs get assigned by reference

Structs get assigned by reference but should be assigned by value.

```golang
type myStruct struct {
	a string
}

x := myStruct{a:"x"}
print(x.a)
y := x
print(y.a)
y.a = "y"
print(x.a)
```

**Current behaviour**
```
x
x
y
```

**Correct behaviour**
```
x
x
x
```