Improvement on exporters of RcppEigen

This patch provides several bug fixes as well as enhancement on the exporters of RcppEigen.

1. In current version of RcppEigen, `MatrixXd x = Rcpp::as<Eigen::MatrixXd>(x_)` will issue a compilation error. This is because the exporter will use brackets to index `x`: `x[i] = x_[i]`. However, `MatrixXd` doesn't support this subsetting strategy, so instead we should use something like `x.data()[i] = x_[i]`.
2. Current exporter for sparse matrix only supports `dgCMatrix`, not `CsparseMatrix`.
3. Add exporter for `Eigen::MappedSparseMatrix<T, Eigen::RowMajor>` from `dgRMatrix`.