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/key_io.cpp

@@ -7,7 +7,7 @@
 #include <base58.h>
 #include <bech32.h>
 #include <script/interpreter.h>
-#include <script/standard.h>
+#include <script/solver.h>
 #include <util/strencodings.h>
 
 #include <algorithm>