Use unordered_map within cpp functions

[ChrisMuir]

I've been playing around with incorporating std::unordered_map into the cpp functions that perform the value merging after the clusters have been generated. I believe I can get a substantial speed up by re-writing these functions to use unordered_map.

[ChrisMuir]

Just pushed the first pass at working unordered_map into the merge steps for key_collision_merge(), in commit f61fb86. Initial results of local benchmarks look great for large inputs, I'm using vectors of company names as test data, comparison is to the timings prior to adding unordered_map:

• vector len 100,000: 0.75s (4x speed up).
• vector len 1,000,000: 12.5s (55x speed up).
• vector len 5,000,000: 53s (276x speed up).

[ChrisMuir]

Pushed edits adding unordered_map to the initial clustering steps and merge steps for n_gram_merge(). Here's local benchmarks, again the comparison is to the timings I got prior to adding unordered_map, and using the same company names test vectors:

• vector len 100,000: 5.63s (3x speed up).
• vector len 1,000,000: 60.5s (31x speed up).
• vector len 5,000,000: 132s (140x speed up).