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/wallet/test/scriptpubkeyman_tests.cpp

@@ -4,7 +4,7 @@
 
 #include <key.h>
 #include <test/util/setup_common.h>
-#include <script/standard.h>
+#include <script/solver.h>
 #include <wallet/scriptpubkeyman.h>
 #include <wallet/wallet.h>
 #include <wallet/test/util.h>