Refactor the Schema class and add some features to it.
Refactor the Schema class and add some features to it.

- Store the field name as part of its properties array.
- Only pass the field array to functions rather than the name too.
- `$field` now refers to the field array and `$fieldname` refers to the field name.
- Reorder the parameters in Schema->addValidator().
- Implement the logic when a Schema->addValidator() is called on an individual field.
vanilla/garden