Add first, nth, last (fix #510)

tests/all.test

@@ -242,6 +242,10 @@ null
 [11,22,33,44,55,66,77,88,99]
 [11,22,33]
 
+[first(range(.)), last(range(.)), nth(0; range(.)), nth(5; range(.)), try nth(-1; range(.)) catch .]
+10
+[0,9,0,5,"nth doesn't support negative indices"]
+
 #
 # Slices
 #