Separate and add benchmark tests

I have separated `starts_with_incremental_length` to
`starts_with_same_vector`, `starts_with_single_element` and
`starts_with_diff_one_element_end` and `ends_with_decremental_length`
to `ends_with_same_vector`, `ends_with_single_element` and
`ends_with_diff_one_element_beginning`. In addition to that, I've
added `contains_last_element` benchamrk test.