Add unwrap_or and unwrap_or_else to NumOpResult

Two useful combinators, add them.

I copied the function signature and docs from stdlib and wrote the
functions myself - thereby operating within licensing requirements.

units/src/result.rs

@@ -142,6 +142,32 @@ impl<T: fmt::Debug> NumOpResult<T> {
         }
     }
 
+    /// Returns the contained Some value or a provided default.
+    ///
+    /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing the result of a
+    /// function call, it is recommended to use `unwrap_or_else`, which is lazily evaluated.
+    #[inline]
+    #[track_caller]
+    pub fn unwrap_or(self, default: T) -> T {
+        match self {
+            R::Valid(x) => x,
+            R::Error(_) => default,
+        }
+    }
+
+    /// Returns the contained `Some` value or computes it from a closure.
+    #[inline]
+    #[track_caller]
+    pub fn unwrap_or_else<F>(self, f: F) -> T
+    where
+        F: FnOnce() -> T,
+    {
+        match self {
+            R::Valid(x) => x,
+            R::Error(_) => f(),
+        }
+    }
+
     /// Converts this `NumOpResult` to an `Option<T>`.
     #[inline]
     pub fn ok(self) -> Option<T> {