Renaming in prelude/list.
Rename: - prefix_of -> prefix and suffix_of -> suffix because that saves keystrokes in lemma names. However, keep the infix notations with l1 `prefix_of` l2 and l1 `suffix_of` l2 because those are easier to read. - change the notation l1 `sublist` l2 into l1 `sublist_of` l2 to be consistent. - rename contains -> submseteq and use the notation ⊆+