Word frequency counter

[danielm-orca]

Implemented in two steps:

1. Get frequencies in a HashMap:
2. Change merged frequencies into BinaryHeap (max heap) and get K=10 most common words

Notes:

• This doesn't conform with most online solutions of frequency, as usually they lowercase everything and then take the frequency.
• Instead of poping from the heap we can use into_iter_sorted, but that's an experimental feature.