refactor: Replace fwd-decl with proper include

This is fine, because the span.h include is lightweight and a proper
include will be needed anyway when switching to std::span.

src/script/solver.h

@@ -10,6 +10,7 @@
 
 #include <attributes.h>
 #include <script/script.h>
+#include <span.h>
 
 #include <string>
 #include <optional>
@@ -17,7 +18,6 @@
 #include <vector>
 
 class CPubKey;
-template <typename C> class Span;
 
 enum class TxoutType {
     NONSTANDARD,

src/test/util/net.h

@@ -12,6 +12,7 @@
 #include <netaddress.h>
 #include <node/connection_types.h>
 #include <node/eviction.h>
+#include <span.h>
 #include <sync.h>
 #include <util/sock.h>
 
@@ -28,9 +29,6 @@
 
 class FastRandomContext;
 
-template <typename C>
-class Span;
-
 struct ConnmanTestMsg : public CConnman {
     using CConnman::CConnman;
 

src/util/hasher.h

@@ -8,13 +8,12 @@
 #include <crypto/common.h>
 #include <crypto/siphash.h>
 #include <primitives/transaction.h>
+#include <span.h>
 #include <uint256.h>
 
 #include <cstdint>
 #include <cstring>
 
-template <typename C> class Span;
-
 class SaltedTxidHasher
 {
 private: