refactor: add most of src/util to iwyu

These files change infrequently, and not much header shuffling is required.

We don't add everything in src/util/ yet, because IWYU makes some
dubious suggestions, which I'm going to follow up with upstream.

src/util/bytevectorhash.cpp

@@ -6,6 +6,8 @@
 #include <random.h>
 #include <util/bytevectorhash.h>
 
+#include <vector>
+
 ByteVectorHash::ByteVectorHash() :
     m_k0(GetRand<uint64_t>()),
     m_k1(GetRand<uint64_t>())