Added string conversion op for amounts and balances

src/amount.h

@@ -496,6 +496,9 @@ public:
   long   to_long() const;
   bool   fits_in_long() const;
 
+  operator string() const {
+    return to_string();
+  }
   string to_string() const;
   string to_fullstring() const;
   string quantity_string() const;

src/balance.h

@@ -461,6 +461,15 @@ public:
    * Conversion methods.  A balance can be converted to an amount, but
    * only if contains a single component amount.
    */
+  operator string() const {
+    return to_string();
+  }
+  string to_string() const {
+    std::ostringstream buf;
+    print(buf);
+    return buf.str();
+  }
+
   amount_t to_amount() const {
     if (is_empty())
       throw_(balance_error, _("Cannot convert an empty balance to an amount"));