Handle list head and tail updates with explicit empty-list errors

fieldglass.mli

@@ -9,3 +9,5 @@ val compose : ('s, 't, 'a, 'b) lens -> ('a, 'b, 'c, 'd) lens -> ('s, 't, 'c, 'd)
 val _1 : ('a * 'c, 'b * 'c, 'a, 'b) lens
 val _2 : ('c * 'a, 'c * 'b, 'a, 'b) lens
 val _id : ('a, 'b, 'a, 'b) lens
+val _hd : ('a list, 'a) lens'
+val _tl : ('a list, 'a list) lens'