Feature Request: Compiler Warning Messages

It would be a nice feature of the CakeML compiler for it to provide warning messages during compilation of possibly unintended behaviors.
 
**Example:**
```sml
fun is_upper_X str =
let
val uppX = "X"
val lowX = "x"
in
case str of
uppX => True
| lowX => False
end

val () =
let
val _ = TextIO.print (Bool.toString (is_upper_X "X"))
val _ = TextIO.print "\n"
val _ = TextIO.print (Bool.toString (is_upper_X "x"))
val _ = TextIO.print "\n"
in
()
end
```
Running it will return
```
True
True (* Possibly unexpected behavior *)
```

I believe it would be beneficial for the compiler to give the user a warning such as `Pattern "lowX" is redundant in this clause.`