Refactor: BLI: Add MutableSpan::contains() function

Add `bool MutableSpan<T>::contains(const T &value)` function, which is an
exact copy of the function from the `Span<T>` class. This makes it possible
to call `.contains()` directly on the `MutableSpan`, instead of having to
call `.as_span()` first.

Note that the `contains_ptr()` function was already copied from `Span` to
`MutableSpan` before.

No functional changes to the affected code, just the removal of the now
no longer necessary `.to_span()` call.

Pull Request: https://projects.blender.org/blender/blender/pulls/131149

source/blender/editors/curves/intern/curves_ops.cc

@@ -1364,7 +1364,7 @@ static int exec(bContext *C, wmOperator * /*op*/)
                             [&](const int i) { cyclic.span[i] = !cyclic.span[i]; });
     cyclic.finish();
 
-    if (!cyclic.span.as_span().contains(true)) {
+    if (!cyclic.span.contains(true)) {
       attributes.remove("cyclic");
     }