Add .cast/int and update .cast/real.

This patch adds .cast/int and updates .cast/real to act as a local
(temporary) net and to support either a signed or unsigned input.
The vvp_vector4_t class not can convert an arbitrarily sized double
to a vector value. This removes the restriction of lround().

Also document the new statements.

target.cc

@@ -107,6 +107,13 @@ bool target_t::lpm_array_dq(const NetArrayDq*)
       return false;
 }
 
+bool target_t::lpm_cast_int(const NetCastInt*)
+{
+      cerr << "target (" << typeid(*this).name() << "): "
+	    "Unhandled NetCastInt." << endl;
+      return false;
+}
+
 bool target_t::lpm_cast_real(const NetCastReal*)
 {
       cerr << "target (" << typeid(*this).name() << "): "