Add Zero Trait

The core or standard library could probably use a Zero trait that allows certain types to provide a zero value.  For instance integers would zero to "0", floats to "0.0", strings to an empty string, bools to false, Options probably to None etc.

What exact types make sense to zero or not is most likely up to discussion.  Related to that it might be useful to have a `get_or_zero` on `Option`.
