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/Makefile.am

@@ -143,7 +143,6 @@ BITCOIN_CORE_H = \
   interfaces/wallet.h \
   key.h \
   key_io.h \
-  keystore.h \
   dbwrapper.h \
   limitedmap.h \
   logging.h \
@@ -184,6 +183,7 @@ BITCOIN_CORE_H = \
   script/keyorigin.h \
   script/sigcache.h \
   script/sign.h \
+  script/signingprovider.h \
   script/standard.h \
   shutdown.h \
   streams.h \
@@ -447,7 +447,6 @@ libbitcoin_common_a_SOURCES = \
   core_write.cpp \
   key.cpp \
   key_io.cpp \
-  keystore.cpp \
   merkleblock.cpp \
   netaddress.cpp \
   netbase.cpp \
@@ -461,6 +460,7 @@ libbitcoin_common_a_SOURCES = \
   scheduler.cpp \
   script/descriptor.cpp \
   script/sign.cpp \
+  script/signingprovider.cpp \
   script/standard.cpp \
   versionbitsinfo.cpp \
   warnings.cpp \