Improve mismatch message in assert_eq for arrays

### What's hard to do? (limit 100 words)

The change at https://github.com/google/xls/commit/f530b769e765910ac6251e403ab449fda4c6ec00 has been very useful. However, I noticed that it does not apply to array elements when comparing arrays.

E.g.

```
assert_eq(
some_fn(...),
u32[4]:[u32:0x75312468, u32:0, ...]);
```

might spit out something like:

```
FailureError: The program being interpreted failed!
lhs and rhs were not equal:
[
< u32:960064620,
> u32:1966154856,
u32:0,
u32:0,
u32:0
]
; first differing index: 0 :: 960064620 vs 1966154856
```

It would be nice if the change to use the literal base could apply to each array element as in the scalar case.

### Current best alternative workaround (limit 100 words)

Open up a Python interpreter (or some online base converter) to convert the decimal values to the form in the original test.

### Your view of the "best case XLS enhancement" (limit 100 words)

If the previous change could be extended to array elements, that would be great.