Custom profile fields allow for shortnames containing mixed character
casing, which should also be supported within the user fields API
when retrieving said custom fields.
Given that the DML libs convert all selected columns to lowercased
equivalent, we need to do the same when requesting custom fields to
ensure calling code doesn't try to access `->profile_field_Foo`
property when the DB returns `->profile_field_foo` instead.
This class would belong more appropriately within the 'user' API
(core_user) instead of within the 'core' API, since it is
directly related to user data.
Since the class has only just been added to Moodle, now is a good
time to move it.
