script: expose getter for CScriptNum, add a BuildScript helper

Some prep work for Miniscript. BuildScript is an efficient way to build
Scripts in a generic manner (by concatenating OPs, data, and other
Scripts).

Co-Authored-By: Pieter Wuille <pieter@wuille.net>

src/script/script.h

@@ -335,6 +335,8 @@ public:
         return m_value;
     }
 
+    int64_t GetInt64() const { return m_value; }
+
     std::vector<unsigned char> getvch() const
     {
         return serialize(m_value);
@@ -578,4 +580,29 @@ bool IsOpSuccess(const opcodetype& opcode);
 
 bool CheckMinimalPush(const std::vector<unsigned char>& data, opcodetype opcode);
 
+/** Build a script by concatenating other scripts, or any argument accepted by CScript::operator<<. */
+template<typename... Ts>
+CScript BuildScript(Ts&&... inputs)
+{
+    CScript ret;
+    int cnt{0};
+
+    ([&ret, &cnt] (Ts&& input) {
+        cnt++;
+        if constexpr (std::is_same_v<std::remove_cv_t<std::remove_reference_t<Ts>>, CScript>) {
+            // If it is a CScript, extend ret with it. Move or copy the first element instead.
+            if (cnt == 0) {
+                ret = std::forward<Ts>(input);
+            } else {
+                ret.insert(ret.end(), input.begin(), input.end());
+            }
+        } else {
+            // Otherwise invoke CScript::operator<<.
+            ret << input;
+        }
+    } (std::forward<Ts>(inputs)), ...);
+
+    return ret;
+}
+
 #endif // BITCOIN_SCRIPT_SCRIPT_H