`print.Class` to handle composition

Composition added in #5, but the print method needs to be updated. 

```R
Class("Foo", a = t_int, b = t_dbl, c = t_char)
Class("Bar", foo = Foo, baz = t_dbl)

foo <- Foo(a = 1L, b = 2.0, c = "xxx")
bar <- Bar(foo = foo, baz = 3.0)
bar
```

```R
Error in as.data.frame.default(x[[i]], optional = TRUE) : 
  cannot coerce class ‘"ClassMap"’ to a data.frame
```