Rename script/standard.{cpp/h} to script/solver.{cpp/h}

Since script/standard only contains things that are used by the Solver
and its callers, rename the files to script/solver.

src/qt/test/wallettests.cpp

@@ -24,7 +24,7 @@
 #include <qt/transactiontablemodel.h>
 #include <qt/transactionview.h>
 #include <qt/walletmodel.h>
-#include <script/standard.h>
+#include <script/solver.h>
 #include <test/util/setup_common.h>
 #include <validation.h>
 #include <wallet/test/util.h>