Array access et al

[SebastianMestre]

We don't currently have a way to write to some position of an array. How can we implement this?

fib := array { 1; 2; 3; 4; };
fib[3] = 5;

I thought of two things:

• One is to add a level of indirection on every array element, by storing it in a Reference. This would be pretty easy and require no changes to how we implement assignment in the interpreter.
• The other is to add a new Reference-like type, but for array elements. It would store a pointer to the array, along with an index. This would be relatively easy and require no changes to how we store arrays in the interpreter.

I prefer the second approach.

An issue I have identified with it is that we would have two reference-like types going around in the interpreter, meaning unboxed() would be slightly more complicated

Also, what a bad name unboxed is! We need a better one. I came up with value_of, off the top of my head, but something else would be good, too.

[SebastianMestre]

I ended up doing the first one in #224