Possible move of comparison to a separate function inside set

``` python
try:
silent = bool(old_value == new_value)
except:
# if there is an error in comparing, default to notify
silent = False
```

When `==` doesn't work (think `np.array` or `pd.DataFrame`), this is equivalent to failing silently in some sense. We should move this section to a separate compare function that can be overload by a `TraitType` so the entire logic of `set` doesn't need to be copied.