Implement ECALL and EBREAK, the final RV64I instructions!

src/decode.rs

@@ -93,4 +93,9 @@ impl Instruction {
     pub fn funct6(self) -> u8 {
         (self.0 >> 26 & 0x3f) as u8
     }
+
+    /// Mostly/only used for the SYSTEM opcode
+    pub fn funct12(self) -> u16 {
+        (self.0 >> 20) as u16
+    }
 }