support attempt for as<field<...>>

This is related to this http://stackoverflow.com/questions/27004232/using-fields-within-rcpparmadillo

With this I get: 

```
$ RcppScript /tmp/arma.cpp

> direct_sum(list(matrix(1:16, nc = 4), matrix(1:16,
+     nc = 4)))
     [,1] [,2]
[1,]    4    4
[2,]    4    4
```