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/verifymessagedialog.cpp

@@ -14,6 +14,7 @@
 #include "walletmodel.h"
 #include "addresstablemodel.h"
 #include "guiutil.h"
+#include "base58.h"
 
 VerifyMessageDialog::VerifyMessageDialog(AddressTableModel *addressModel, QWidget *parent) :
     QDialog(parent),
@@ -62,7 +63,7 @@ bool VerifyMessageDialog::checkAddress()
         return false;
     }
 
-    CBitcoinAddress address(key.GetPubKey());
+    CBitcoinAddress address(key.GetPubKey().GetID());
     QString qStringAddress = QString::fromStdString(address.ToString());
     ui->lnAddress->setText(qStringAddress);
     ui->copyToClipboard->setEnabled(true);