Expose maximum array length

[ppolesiuk]

In a discussion @ppolesiuk suggested that we could take an approach similar to OCaml and expose the maximum array length for when the programmer requires this kind of assurance. I think this is reasonable, given that oversized allocations are uncommon and recovery from out of memory situations is rarely needed/desirable (at least for such a high level language). However, perhaps we still want to check the size somewhere, to display a custom error instead of OCaml's.

Originally posted by @forell in #126 (comment)