Move user_fields from core to core_user

There is a new "utility class" in code (MDL-45242), \core\user_fields for 311 and up.

After some discussion @ developer chat it was commented that, maybe, it had more sense to make that class part of the user subsystem.

That will imply moving it to be \core_user\user_fields (or maybe \core_user\fields ??) and, also, switch all the current uses in core to the new class.

No need for class aliasing should be applied as far as the affected versions are, all them, dev versions.

So this issue is about:

1) Decide if we perform the move.
2) Apply for it if agreed to.

Ciao

PS: Also, it was agreed that, in any case, the new class is not an API per se (it's not going to be implemented or extended over other components), so just a nice util class (like core_text and others...).