check task priority at compile time

before we were checking the priority at runtime. The compile time error message
when the priority is too high is kind of awful though.

tests/cfail/priority-too-high.rs

@@ -0,0 +1,22 @@
+#![no_main]
+#![no_std]
+
+extern crate lm3s6965;
+extern crate panic_halt;
+extern crate rtfm;
+
+use rtfm::app;
+
+#[app(device = lm3s6965)] //~ error evaluation of constant value failed
+const APP: () = {
+    #[init]
+    fn init() {}
+
+    // OK, this is the maximum priority supported by the device
+    #[interrupt(priority = 8)]
+    fn UART0() {}
+
+    // this value is too high!
+    #[interrupt(priority = 9)]
+    fn UART1() {}
+};

tests/cfail/priority-too-low.rs

@@ -0,0 +1,22 @@
+#![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() {}
+
+    // OK, this is the minimum priority that tasks can have
+    #[interrupt(priority = 1)]
+    fn UART0() {}
+
+    // this value is too low!
+    #[interrupt(priority = 0)] //~ error this literal must be in the range 1...255
+    fn UART1() {}
+};