feat: implement M3 aggregate types and iteration

fec/tests/m3/array.fe

@@ -0,0 +1,9 @@
+unit m3_array;
+
+fn main() -> i32 {
+    let a: [3]i32 = [1, 2, 3];
+    let s: []i32 = a[..];
+    let t: []i32 = s[1..3];
+    if a[0] + s[1] + t[0] == 5 and s.n == 3 { return 0; }
+    return 1;
+}