support struct patterns with partial defaults (resolves #35)

test/basic/struct_default.sv

@@ -0,0 +1,11 @@
+module top;
+    typedef struct packed {
+        logic a;
+        logic b;
+    } foo_t;
+    foo_t foo;
+    initial begin
+        foo = '{a: 1'b1, default: '0};
+        $display(foo, foo.a, foo.b);
+    end
+endmodule

test/basic/struct_default.v

@@ -0,0 +1,4 @@
+module top;
+    reg [1:0] foo = {1'b1, 1'b0};
+    initial $display(foo, foo[1], foo[0]);
+endmodule