support const declarations of alias types

test/basic/const.sv

@@ -0,0 +1,10 @@
+module top;
+    typedef struct packed { integer y, z; } T;
+    // TODO iverilog doesn't allow references to variables in initializers
+    // if (1) begin : blk
+    // integer a = 11;
+    // integer b = 12;
+    const integer w = 11;
+    const T x = '{ y: 11, z: 12 };
+    initial $display("%b %b %b %b", w, x, x.y, x.z);
+endmodule

test/basic/const.v

@@ -0,0 +1,5 @@
+module top;
+    integer w = 11;
+    wire [63:0] x = { 32'd11, 32'd12 };
+    initial $display("%b %b %b %b", w, x, x[32+:32], x[0+:32]);
+endmodule