Support math expressions as array indexes

src/parser.rs

@@ -1449,7 +1449,7 @@ impl Parser {
                 // Indexing operation
                 self.next();
 
-                let item = self.simple_expression();
+                let item = self.expression();
                 let span_end = self.position() + 1;
                 self.rsquare();
 

tests/integration/math/math_in_brackets.june

@@ -0,0 +1,7 @@
+// output: 23
+mut data = raw[1,2,3]
+unsafe {
+    println(data[1])
+    data[0 + 1] = 3
+    println(data[1])
+}