Refactor: split CKeyID/CScriptID/CTxDestination from CBitcoinAddress

This introduces internal types:
* CKeyID: reference (hash160) of a key
* CScriptID: reference (hash160) of a script
* CTxDestination: a boost::variant of the former two

CBitcoinAddress is retrofitted to be a Base58 encoding of a
CTxDestination. This allows all internal code to only use the
internal types, and only have RPC and GUI depend on the base58 code.

Furthermore, the header dependencies are a lot saner now. base58.h is
at the top (right below rpc and gui) instead of at the bottom. For the
rest: wallet -> script -> keystore -> key. Only keystore still requires
a forward declaration of CScript. Solving that would require splitting
script into two layers.

src/key.h

@@ -44,6 +44,23 @@ public:
     explicit key_error(const std::string& str) : std::runtime_error(str) {}
 };
 
+/** A reference to a CKey: the Hash160 of its serialized public key */
+class CKeyID : public uint160
+{
+public:
+    CKeyID() : uint160(0) { }
+    CKeyID(const uint160 &in) : uint160(in) { }
+};
+
+/** A reference to a CScript: the Hash160 of its serialization (see script.h) */
+class CScriptID : public uint160
+{
+public:
+    CScriptID() : uint160(0) { }
+    CScriptID(const uint160 &in) : uint160(in) { }
+};
+
+/** An encapsulated public key. */
 class CPubKey {
 private:
     std::vector<unsigned char> vchPubKey;
@@ -60,8 +77,8 @@ public:
         READWRITE(vchPubKey);
     )
 
-    uint160 GetID() const {
-        return Hash160(vchPubKey);
+    CKeyID GetID() const {
+        return CKeyID(Hash160(vchPubKey));
     }
 
     uint256 GetHash() const {
@@ -72,6 +89,10 @@ public:
         return vchPubKey.size() == 33 || vchPubKey.size() == 65;
     }
 
+    bool IsCompressed() const {
+        return vchPubKey.size() == 33;
+    }
+
     std::vector<unsigned char> Raw() const {
         return vchPubKey;
     }