Support string concat and concat-head operations

Doctor's orders: !938 (comment 14975)

examples/string_ops.ion

+let string = world
+echo $string
+let string ++= !
+echo $string
+let string ::= "Hello, "
+echo $string
+
+let string1 = Doctor
+let string2 = order
+echo $string1
+echo $string2
+let string1 ++= "'s"
+let string2 ++= "s"
+echo $string1
+echo $string2
+let string1 ++= " "
+let string2 ::= $string1
+echo $string2