Add len() std function

lib/stdio.sb

@@ -7,4 +7,14 @@ fn print(msg) {
 // Like print(), but with an extra newline ('\n') character
 fn println(msg) {
     print(msg + "\n")
+}
+
+// Prints the size of an array
+fn len(arr) {
+    let c = 0
+    while arr[c] {
+        c = c + 1
+    }
+
+    return c
 }