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/test/sigopcount_tests.cpp

@@ -10,7 +10,7 @@
 #include <pubkey.h>
 #include <script/interpreter.h>
 #include <script/script.h>
-#include <script/standard.h>
+#include <script/solver.h>
 #include <test/util/setup_common.h>
 #include <uint256.h>