Add support decoding into nested interfaces.
Add support decoding into nested interfaces.

This support is dependent on the interface containing a concrete type and
a value which is addressable and settable.

nullstyle/go-xdr,dajohi/go-xdr,postfix/go-xdr,dajohi/go-xdr,rasky/go-xdr,davecgh/go-xdr,nullstyle/go-xdr,rasky/go-xdr,davecgh/go-xdr,postfix/go-xdr