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/multisig_tests.cpp

@@ -3,12 +3,12 @@
 // file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include <key.h>
-#include <keystore.h>
 #include <policy/policy.h>
 #include <script/script.h>
 #include <script/script_error.h>
 #include <script/interpreter.h>
 #include <script/sign.h>
+#include <script/signingprovider.h>
 #include <tinyformat.h>
 #include <uint256.h>
 #include <test/setup_common.h>