Allow to trace expressions (and not only string literals)

This however enforces that the argument unifies to a `String`. So this
  is more flexible than the previous form, but does fundamentally the
  same thing.

  Fixes #378.

examples/acceptance_tests/032/lib/tests.ak

@@ -1,9 +1,19 @@
+use aiken/builtin
+
+fn concat(left: String, right: String) -> String {
+  builtin.append_bytearray(
+    builtin.encode_utf8(left),
+    builtin.encode_utf8(right),
+  )
+  |> builtin.decode_utf8
+}
+
 fn is_negative(i: Int) -> Bool {
   if i < 0 {
     trace("is negative")
     True
   } else {
-    trace("is non-negative")
+    trace(concat("is", concat(" ", "non-negative")))
     False
   }
 }