Auto-format integer constants used in tie-offs

For the autowrap feature, it would be nice if tieoffs like this worked:

```python
tieoffs = {
    'signal1': dict(value=42, width=8),
    'signal2': dict(value=0xcafe, width=16, format='hex')
}
```

generating something like this:

```verilog
.signal1(8'd42),
.signal2(16'hcafe)
```
