New fixed arithmetic tests

pid/tests/fixed.rs

+use pid::Fixed;
+
+type F16 = Fixed<16>;
+
+#[test]
+fn round() {
+    let v = F16::from_raw(0x10000);
+    assert_eq!(v.round(), 1);
+    let v = F16::from_raw(0xffff);
+    assert_eq!(v.round(), 1);
+    let v = F16::from_raw(0x8000);
+    assert_eq!(v.round(), 1);
+    let v = F16::from_raw(0x7fff);
+    assert_eq!(v.round(), 0);
+    let v = F16::from_raw(-0x10000);
+    assert_eq!(v.round(), -1);
+    let v = F16::from_raw(-0xffff);
+    assert_eq!(v.round(), -1);
+    let v = F16::from_raw(-0x8000);
+    assert_eq!(v.round(), -1);
+    let v = F16::from_raw(-0x7fff);
+    assert_eq!(v.round(), 0);
+}