Support string concat and concat-head operations

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

examples/string_ops.out

+world
+world!
+Hello, world!
+Doctor
+order
+Doctor's
+orders
+Doctor's orders

src/lib/shell/mod.rs

@@ -474,9 +474,18 @@ impl Shell {
         match lhs {
             Value::Str(lhs) => {
                 if let Value::Str(rhs) = rhs {
-                    let action = math(&key.kind, operator, &rhs).map_err(|why| why.to_string())?;
-                    let value = parse(&lhs, &*action).map_err(|why| why.to_string())?;
-                    *lhs = value.into();
+                    match operator {
+                        Operator::Concatenate => lhs.push_str(&rhs),
+                        Operator::ConcatenateHead => {
+                            *lhs = rhs + lhs;
+                        }
+                        _ => {
+                            let action =
+                                math(&key.kind, operator, &rhs).map_err(|why| why.to_string())?;
+                            let value = parse(&lhs, &*action).map_err(|why| why.to_string())?;
+                            *lhs = value.into();
+                        }
+                    }
                 }
                 Ok(())
             }