Str.concat tests and definition set up

compiler/module/src/low_level.rs

@@ -3,6 +3,7 @@
 /// into an Expr when added directly by can::builtins
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub enum LowLevel {
+    StrConcat,
     ListLen,
     ListGetUnsafe,
     ListSet,

compiler/module/src/symbol.rs

@@ -652,6 +652,7 @@ define_builtins! {
         1 STR_AT_STR: "@Str" // the Str.@Str private tag
         2 STR_ISEMPTY: "isEmpty"
         3 STR_APPEND: "append"
+        4 STR_CONCAT: "concat"
     }
     4 LIST: "List" => {
         0 LIST_LIST: "List" imported // the List.List type alias