reject duplicate arguments in #[interrupt] and #[exception]

This program was being accepted:

``` rust
 #[task(
    capacity = 1,
    capacity = 2,
    priority = 1,
    priority = 2,
)]
fn foo() {}
```

now it will trigger a compiler error

tests/cfail/duplicate-args-2.rs

@@ -0,0 +1,24 @@
+#![no_main]
+#![no_std]
+
+extern crate lm3s6965;
+extern crate panic_halt;
+extern crate rtfm;
+
+use rtfm::app;
+
+#[app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn init() {}
+
+    #[task(
+        priority = 1,
+        priority = 2, //~ ERROR argument appears more than once
+    )]
+    fn foo() {}
+
+    extern "C" {
+        fn UART0();
+    }
+};

tests/cfail/duplicate-args.rs

@@ -0,0 +1,24 @@
+#![no_main]
+#![no_std]
+
+extern crate lm3s6965;
+extern crate panic_halt;
+extern crate rtfm;
+
+use rtfm::app;
+
+#[app(device = lm3s6965)]
+const APP: () = {
+    #[init]
+    fn init() {}
+
+    #[task(
+        capacity = 1,
+        capacity = 2, //~ ERROR argument appears more than once
+    )]
+    fn foo() {}
+
+    extern "C" {
+        fn UART0();
+    }
+};