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/qt/messagepage.cpp

@@ -10,6 +10,7 @@
 #include "main.h"
 #include "wallet.h"
 #include "init.h"
+#include "base58.h"
 
 #include "messagepage.h"
 #include "ui_messagepage.h"
@@ -83,6 +84,13 @@ void MessagePage::on_signMessage_clicked()
                               QMessageBox::Abort, QMessageBox::Abort);
         return;
     }
+    CKeyID keyID;
+    if (!addr.GetKeyID(keyID))
+    {
+        QMessageBox::critical(this, tr("Error signing"), tr("%1 does not refer to a key.").arg(address),
+                              QMessageBox::Abort, QMessageBox::Abort);
+        return;
+    }
 
     WalletModel::UnlockContext ctx(model->requestUnlock());
     if(!ctx.isValid())
@@ -92,7 +100,7 @@ void MessagePage::on_signMessage_clicked()
     }
 
     CKey key;
-    if (!pwalletMain->GetKey(addr, key))
+    if (!pwalletMain->GetKey(keyID, key))
     {
         QMessageBox::critical(this, tr("Error signing"), tr("Private key for %1 is not available.").arg(address),
                               QMessageBox::Abort, QMessageBox::Abort);