AK: Declare operators new and delete as global functions

This fixes a build issue introduced in 23d66fe, where the compiler
statically detected that that mismatching new and delete operators were
used.

Clang generates a warning for this, for the reasons described in the
comment in `AK/kmalloc.cpp`, but GCC does not.

Besides moving the allocator functions into a `.cpp` file, declarations
in `AK/kmalloc.cpp` were reordered to have imports at the top, in order
to make the code more readable.

AK/kmalloc.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2021, Daniel Bertalan <dani@danielbertalan.dev>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#if defined(__serenity__) && !defined(KERNEL)
+
+#    include <AK/Assertions.h>
+#    include <AK/kmalloc.h>
+
+// However deceptively simple these functions look, they must not be inlined.
+// Memory allocated in one translation unit has to be deallocatable in another
+// translation unit, so these functions must be the same everywhere.
+// By making these functions global, this invariant is enforced.
+
+void* operator new(size_t size)
+{
+    void* ptr = malloc(size);
+    VERIFY(ptr);
+    return ptr;
+}
+
+void* operator new(size_t size, const std::nothrow_t&) noexcept
+{
+    return malloc(size);
+}
+
+void operator delete(void* ptr) noexcept
+{
+    return free(ptr);
+}
+
+void operator delete(void* ptr, size_t) noexcept
+{
+    return free(ptr);
+}
+
+void* operator new[](size_t size)
+{
+    void* ptr = malloc(size);
+    VERIFY(ptr);
+    return ptr;
+}
+
+void* operator new[](size_t size, const std::nothrow_t&) noexcept
+{
+    return malloc(size);
+}
+
+void operator delete[](void* ptr) noexcept
+{
+    return free(ptr);
+}
+
+void operator delete[](void* ptr, size_t) noexcept
+{
+    return free(ptr);
+}
+
+#endif