Allow casting to/constructing of array type

[matt-attack]

Currently there is no way to instantiate an array type without also allocating an array. Perhaps allow user to instantiate on the stack a [] object and assign to it manually.

let int[] arr;
arr.size = 5;
arr.ptr = <int*>malloc(5*4);

More complex syntax could also be allowed which lets the user construct it like any other object.

let arr = int[](5, <int*>malloc(5*4));

[matt-attack]

It's possible the first case is already workable, but it doesn't initialize the values to anything and is a potential safety issue. If that's true, just work on the second type.