Implement ECALL and EBREAK, the final RV64I instructions!

src/exceptions.rs

@@ -31,6 +31,15 @@ pub enum ExceptionType {
     HardwareError = 19,
 }
 
+impl ExceptionType {
+    pub fn with_no_value(self) -> Exception {
+        Exception {
+            type_: self,
+            value: 0,
+        }
+    }
+}
+
 #[derive(Debug, Clone, Copy)]
 pub struct Exception {
     pub type_: ExceptionType,