Replace some use of variant with covariant (#322)

src/arc-mutex/arc-layout.md

@@ -47,7 +47,7 @@ all the details on variance and drop check.
 To fix the first problem, we can use `NonNull<T>`. Note that `NonNull<T>` is a
 wrapper around a raw pointer that declares that:
 
-* We are variant over `T`
+* We are covariant over `T`
 * Our pointer is never null
 
 To fix the second problem, we can include a `PhantomData` marker containing an

src/phantom-data.md

@@ -39,7 +39,7 @@ struct Iter<'a, T: 'a> {
 }
 ```
 
-and that's it. The lifetime will be bounded, and your iterator will be variant
+and that's it. The lifetime will be bounded, and your iterator will be covariant
 over `'a` and `T`. Everything Just Works.
 
 Another important example is Vec, which is (approximately) defined as follows:

src/subtyping.md

@@ -443,7 +443,7 @@ struct MyType<'a, 'b, A: 'a, B: 'b, C, D, E, F, G, H, In, Out, Mixed> {
     f: Vec<F>,    // covariant over F
     g: Cell<G>,   // invariant over G
 
-    h1: H,        // would also be variant over H except...
+    h1: H,        // would also be covariant over H except...
     h2: Cell<H>,  // invariant over H, because invariance wins all conflicts
 
     i: fn(In) -> Out,       // contravariant over In, covariant over Out