support complex sizes in size casts (resolves #69)

test/basic/size_cast.sv

@@ -1,6 +1,15 @@
 module top;
     localparam BW = 3;
     logic [2:0] test;
-    assign test = BW'(0);
-    initial #1 $display(test);
+    logic [3:0] foo;
+    logic [3:0] bar;
+
+    initial begin
+        test = BW'(0);
+        $display(test);
+        foo = 2'('1);
+        $display(foo);
+        bar = $bits(bar)'('1);
+        $display(bar);
+    end
 endmodule

test/basic/size_cast.v

@@ -1,6 +1,14 @@
 module top;
-    localparam BW = 3;
-    wire [2:0] test;
-    assign test = 0;
-    initial #1 $display(test);
+    reg [2:0] test;
+    reg [3:0] foo;
+    reg [3:0] bar;
+
+    initial begin
+        test = 0;
+        $display(test);
+        foo = 4'b0011;
+        $display(foo);
+        bar = 4'b1111;
+        $display(bar);
+    end
 endmodule