fix slop

[PSeitz]

Fix slop by carrying slop so far for terms > 2.
Define slop contract in the API

• The query will match if its terms are separated by slop terms at most.
  The slop can be considered a budget between all terms.
  E.g. "A B C" with slop 1 allows "A X B C", "A B X C", but not "A X B X C".

• Transposition costs 2, e.g. "A B" with slop 1 will not match "B A" but it would with slop 2
  Transposition is not a special case, in the example above A is moved 1 position and B is moved 1 position, so the slop is 2.

• As a result slop works in both directions, so the order of the terms may changed as long as they respect the slop.

The slop carrying algorithm is only active for terms > 2 as it is slightly slower

running 6 tests
test query::phrase_query::phrase_scorer::bench::bench_intersection_count_short                                           ... bench:           2 ns/iter (+/- 0)
test query::phrase_query::phrase_scorer::bench::bench_intersection_medium                                                ... bench:         106 ns/iter (+/- 1)
test query::phrase_query::phrase_scorer::bench::bench_intersection_medium_slop                                           ... bench:         104 ns/iter (+/- 7)
test query::phrase_query::phrase_scorer::bench::bench_intersection_medium_slop_carrying                                  ... bench:         194 ns/iter (+/- 3)
test query::phrase_query::phrase_scorer::bench::bench_intersection_short                                                 ... bench:           3 ns/iter (+/- 0)
test query::phrase_query::phrase_scorer::bench::bench_intersection_short_slop                                            ... bench:          16 ns/iter (+/- 0)

[fulmicoton]

awesome explanation