Optimize _field_invoke()

Attached patch refactors _field_invoke() and _field_invoke_multiple() :
- drops earlier if there is nothing to call for a given instance (field type module doesn't implement the corresponding hook_field_$op() hook)
- less array looping on languages
- less array looping on $instances in single-field mode
- as a side bonus, factors out code to determine the list of relevant instances to a helper _field_invoke_get_instances() function that filters according to the $options (do we want to operate on deleted fields as well, do we want to operate on a single field only).