Move various SigningProviders to signingprovider.{cpp,h}

Moves all of the various SigningProviders out of sign.{cpp,h} and
keystore.{cpp,h}. As such, keystore.{cpp,h} is also removed.

Includes and the Makefile are updated to reflect this. Includes were largely
changed using:
git grep -l "keystore.h" | xargs sed -i -e 's;keystore.h;script/signingprovider.h;g'

src/test/txvalidationcache_tests.cpp

@@ -8,8 +8,8 @@
 #include <txmempool.h>
 #include <script/standard.h>
 #include <script/sign.h>
+#include <script/signingprovider.h>
 #include <test/setup_common.h>
-#include <keystore.h>
 
 #include <boost/test/unit_test.hpp>