Allow constant expressions in [Type * n].

So this is a partial fix for #3469. Partial because it only works for simple constant expressions like `32/2` and `2+2` and not for any actual constants.

For example:
```
const FOO: uint = 2+2;
let v: [int * FOO];
```

results in:
```
error: expected constant expr for vector length: Non-constant path in constant expr
```

This seems to be because at the point of error (`typeck::astconv`) the def_map doesn't contain the constant and thus it can't lookup the actual expression (`2+2` in this case).

So, feedback on what I have so far and suggestions for how to address the constant issue?