Introduce interface for signing providers

CKeyStore is a rich interface that provides many features, including knowledge
of scripts and pubkeys for solving, private keys for signing, in addition to
watch-only keys and scripts, and distinguishing lack of keys from them just
being encrypted.

The signing logic in script/sign does not actually need most of these features.
Here we introduce a simpler interface (SigningProvider) which *only* provides
keys and scripts. This is actually sufficient for signing.

In addtion, we swap the dependency between keystore and script/sign
(keystore now depends on script/script with CKeyStore deriving from
SigningProvider, rather than CKeyStore being the interface that signing
relies on).

src/keystore.h

@@ -9,35 +9,31 @@
 #include <key.h>
 #include <pubkey.h>
 #include <script/script.h>
+#include <script/sign.h>
 #include <script/standard.h>
 #include <sync.h>
 
 #include <boost/signals2/signal.hpp>
 
 /** A virtual base class for key stores */
-class CKeyStore
+class CKeyStore : public SigningProvider
 {
 protected:
     mutable CCriticalSection cs_KeyStore;
 
 public:
-    virtual ~CKeyStore() {}
-
     //! Add a key to the store.
     virtual bool AddKeyPubKey(const CKey &key, const CPubKey &pubkey) =0;
     virtual bool AddKey(const CKey &key);
 
     //! Check whether a key corresponding to a given address is present in the store.
     virtual bool HaveKey(const CKeyID &address) const =0;
-    virtual bool GetKey(const CKeyID &address, CKey& keyOut) const =0;
     virtual std::set<CKeyID> GetKeys() const =0;
-    virtual bool GetPubKey(const CKeyID &address, CPubKey& vchPubKeyOut) const =0;
 
     //! Support for BIP 0013 : see https://github.com/bitcoin/bips/blob/master/bip-0013.mediawiki
     virtual bool AddCScript(const CScript& redeemScript) =0;
     virtual bool HaveCScript(const CScriptID &hash) const =0;
     virtual std::set<CScriptID> GetCScripts() const =0;
-    virtual bool GetCScript(const CScriptID &hash, CScript& redeemScriptOut) const =0;
 
     //! Support for Watch-only addresses
     virtual bool AddWatchOnly(const CScript &dest) =0;