Merge bitcoin/bitcoin#28579: refactor: Remove redundant checks in compat/assumptions.h

fa1a384706 Move compat.h include from system.h to system.cpp (MarcoFalke)
88887531b7 Move compat/assumptions.h include to one place that actually needs it (MarcoFalke)
77774110f4 Remove __cplusplus from compat/assumptions.h (MarcoFalke)
faa3d4f1d8 Remove duplicate NDEBUG check from compat/assumptions.h (MarcoFalke)

Pull request description:

  Generally, compile-time checks should be close to the code that use them. Especially, since `compat/assumptions.h` is only included in one place, where iwyu suggests to remove it.

  Fix all issues:
  * The `NDEBUG` check is used in `util/check`, so it is redundant in `compat/assumptions.h`.
  * The `__cplusplus` check is redundant with `doc/dependencies.md` (see commit message).
  * Add missing `// IWYU pragma: keep` to avoid removing the include by accident.

ACKs for top commit:
  achow101:
    ACK fa1a384706
  TheCharlatan:
    re-ACK fa1a384706
  theuni:
    ACK fa1a384706

Tree-SHA512: f8b6db84be5d8844a2267345c0b1405fcbc39b8b5eeaa24db5b8412a74145fe44cf188b6b0c39cc2b062690ed37ca5b4662473484afe28dbec6469e79961389b

src/serialize.h

@@ -7,7 +7,10 @@
 #define BITCOIN_SERIALIZE_H
 
 #include <attributes.h>
+#include <compat/assumptions.h> // IWYU pragma: keep
 #include <compat/endian.h>
+#include <prevector.h>
+#include <span.h>
 
 #include <algorithm>
 #include <cstdint>
@@ -18,13 +21,9 @@
 #include <memory>
 #include <set>
 #include <string>
-#include <string.h>
 #include <utility>
 #include <vector>
 
-#include <prevector.h>
-#include <span.h>
-
 /**
  * The maximum size of a serialized object in bytes or number of elements
  * (for eg vectors) when the size is encoded as CompactSize.