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/bench/ccoins_caching.cpp

@@ -5,7 +5,7 @@
 #include <bench/bench.h>
 #include <coins.h>
 #include <policy/policy.h>
-#include <wallet/crypter.h>
+#include <script/signingprovider.h>
 
 #include <vector>