fix struct typing of ternary expressions (resolves #73)

test/basic/struct_tern.sv

@@ -0,0 +1,12 @@
+module Example(flag, out);
+    typedef struct packed {
+        logic a, b;
+    } T;
+    output T out;
+    input logic flag;
+    assign out =
+        flag
+        ? '{ a: 1'b1, b: 1'b0 }
+        : '{ a: 1'b1, b: 1'b1 }
+        ;
+endmodule

test/basic/struct_tern.v

@@ -0,0 +1,5 @@
+module Example(flag, out);
+    output wire [1:0] out;
+    input wire flag;
+    assign out = flag ? 2'b10 : 2'b11;
+endmodule

test/basic/struct_tern_tb.v

@@ -0,0 +1,12 @@
+module top;
+    reg flag;
+    wire [1:0] out;
+    Example example(flag, out);
+    initial begin
+        $monitor("%2d %b %b", $time, flag, out);
+        #1 flag = 0;
+        #1 flag = 1;
+        #1 flag = 0;
+        #1 flag = 1;
+    end
+endmodule