Error in bed_shuffle() Issue176

[kriemo]

This PR contains a reimplementation of merge_impl which fixes #176

• The reimplementation is just as fast as the previous implementation
• The tests for bed_cluster() were testing for the error, so I fixed the test
• I also added an additional test for bed_merge() for contained intervals

x <- tibble::tribble(
    ~chrom, ~start, ~end,
    'chr1',  1,     10,
    'chr1',  2,     5,
    'chr1',  7,     9
  )

bed_merge(x)
#>  # A tibble: 1 × 3
#>    chrom start   end
#>   <chr> <dbl> <dbl>
#>   1  chr1     1    10