Elaborate nested l-values.

This gets nested l-values to (but just short of) the ivl_target API.
Now the elaborator can process nested l-values, but I haven't figured
out how to present that at the ivl_target.h API.

target.h

@@ -46,8 +46,12 @@ struct target {
  */
 
 struct target_t {
+      inline target_t() : errors(0) { }
       virtual ~target_t();
 
+	/* Set this to count errors encountered during emit. */
+      int errors;
+
 	/* Start the design. This sets the main output file stream
 	   that the target should use. */
       virtual bool start_design(const Design*) =0;