Improve efficiency of sparse queries

[johnlees]

Use a priority queue while building instead of a sort

Also replacing robin_hood map with newer https://github.com/martinus/unordered_dense

TODO: Now fixed, braces were simply in the wrong place, error unfortunately reported confusing location
Getting an error

Progress (CPU): 0.1%terminate called after throwing an instance of 'std::runtime_error'
  what():  Kmer length 14 not found in sketch SAMD00083451

Happens on different samples, can't pin down with printf or gdb.

Wondering if there's a segfault somewhere else, may need valgrind

[nickjcroucher]

Looks brilliant, just a couple of queries, thanks!