AK: Add a constructor from Span for FixedArray

This is particularly useful in the Kernel, where the physical pages of
a VMObject are stored as a FixedArray but often passed around as a Span
from which a new FixedArray should be cloned.

AK/FixedArray.h

@@ -55,6 +55,19 @@ public:
         return FixedArray<T>(N, elements);
     }
 
+    template<typename U>
+    static ErrorOr<FixedArray<T>> try_create(Span<U> span)
+    {
+        if (span.size() == 0)
+            return FixedArray<T>();
+        T* elements = static_cast<T*>(kmalloc_array(span.size(), sizeof(T)));
+        if (!elements)
+            return Error::from_errno(ENOMEM);
+        for (size_t i = 0; i < span.size(); ++i)
+            new (&elements[i]) T(span[i]);
+        return FixedArray<T>(span.size(), elements);
+    }
+
     ErrorOr<FixedArray<T>> try_clone() const
     {
         if (m_size == 0)