Word renaming

Several words have been renamed and moved around to make Factor more consistent:

• new -> new-sequence
• construct-empty -> new
• construct-boa -> boa
• diff -> assoc-diff
• union -> assoc-union
• intersect -> assoc-intersect
• seq-diff -> diff
• seq-intersect -> intersect

To make things symmetrical, a new word union operates on sequences.

Somehow, seq-diff and seq-intersect were implemented as O(n^2) algorithms. Now, they use hashtables and are O(n).

Lastly, a new vocabulary named ``sets'' contains the set theoretic words, along with a new word unique that converts a sequence to a hash table whose keys and values are the same. An efficient union and intersect are implemented in terms of this word.