Added sparse-map to core with corresponding test.

Added a function `sparse-map` which takes a seq and turns it into a map suitable for use as a feature.  Since the sparse format is [here to stay] in libsvm, a utility to convert from a dense format would be useful.

[here to stay]: http://www.csie.ntu.edu.tw/~cjlin/libsvm/faq.html#f303