Fixes to sugar table

Summary:
* Changes the REALSXP -> STRSXP coercion a bit to better match R
* In the case that a `std::map` is used for the mapping, we make sure to supply a comparator that inserts in such a way that the result is sorted as desired. This is done with a set of ugly `#define`s but it is functional. Comparators are in `mapcompare.h`.
* Added checks for `<unordered_map>`, `<unordered_set>` to `capabilities`.

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