Pancake array support

As discussed it might be a good idea to support c-like arrays in pancake. For example
```c
var arr_ptr = @base;
var offset = 4 * 8;
stw arr_ptr + offset, 42;
var x = lds 1 arr_ptr + offset;
```
can be written as
```c
var arr_ptr = @base;
arr_ptr[4] = 42;
var x = arr_ptr[4];
```
That'll certainly make implementation easier and benefit code readability. And potentially also make it easier to translate to ATP intermediate language like silver.