HRW library supports weighted sorting. Weights must be normalized
before applying. Since there could be different types of normalization
for multiple criteria, there is no point to perform simple
normalization in this library. Pass a slice of normalized weights
to the `SortByWeight` functions.
This commit proposes to:
- remove normalization routine from `SortByWeight` function;
- add `ValidateWeights` function to check if weights are normalized;
- rename `weight` -> `distance` to avoid naming confusion between
hash distance and actual weights;
- use testify lib in the tests;
