[DSLX] Switch multidimensional arrays to Rustic syntax?

As mentioned in #526 there is an outstanding discrepancy between DSL multidimensional arrays and Rustic ones. I do think the DSL ones we formulated this way before we got serious about making it as close to Rust as possible (back then we were trying to make it look more like parameterized and concise IR), but some of the nice properties we were thinking at the time we around the inside-out nature of unwrapping the type definition with indexing, e.g. when mixing tuples and arrays:

```
let x: ((u2, u3)[4], (u6, u8)[2][5])[3]
```

In that sort of type definition you unravel the outermost entities to the innermost ones, e.g. `x[0][1][4][1][1]` gets you the `u8` because you're always unwrapping the "rightmost" decorating dimension.

I /believe/ there's no difference in power or clarity in the Rust array syntax (Rust just nests the multidimensional array items more verbosely) but was discussing this with @RobSpringer recently so CC him for his thoughts as well.