hashtables: add a 2hashtable word that is like associate but for two key/value pairs.

core/hashtables/hashtables.factor

@@ -150,6 +150,9 @@ M: hashtable set-at
     [ 2dup hash@ set-nth-pair ] keep
     hashtable boa ; inline
 
+: 2hashtable ( value0 key0 value1 key1 -- hash )
+    H{ } clone [ set-at ] keep [ set-at ] keep ; inline
+
 <PRIVATE
 
 : collect-pairs ( hash quot: ( key value -- elt ) -- seq )