Add compiler support for `celldefine, vpiCellInstance

This patch adds support to the compiler and stub backend for
`celldefine. This matches the compiler portion of the full
support that was added to 0.10.devel. This does not change
vvp or the vvp interface since changing the interface is not
allowed in a stable branch.

Module.h

@@ -74,6 +74,8 @@ class Module : public PScope, public LineInfo {
 	   other effect. */
       bool library_flag;
 
+      bool is_cell;
+
       NetNet::Type default_nettype;
 
 	/* specparams are simpler then other params, in that they have