substitute RCPP_UNORDERED_MAP with std::unordered_map

[Enchufa2]

Dear jiebaR maintainer,

As part of the effort to clean up old code and set C++11 as the baseline standard for Rcpp (see RcppCore/Rcpp#1365), we identified that this package is using RPP_UNORDERED_MAP, and we would like to suggest substituting this define with the proper std::unordered_map type. In this way, we would be able to remove this unnecessary define without breaking your package on CRAN.

We would appreciate if you could merge this PR and send an update to CRAN at your earliest convenience.

Thanks,
On behalf of the Rcpp Core Team