Avoid capturing the block used for sorting, allowing automatic
unpacking of Tuples.

Since the block is only required in the inner `#quicksort!` function,
this change gives some flexibility to the user allowing them write
more expressive code.

In the following example:

    a = [{"c", 3}, {"a", 1}, {"b", 2}]
    a.sort_by &.[1]
    a.sort_by { |(x, y)| y }

Both `sort_by` calls produce the same result, one being more cryptic
and the other being explicit about unpacking.

In comparison:

    a.sort_by { |x, y| y }

Produces the same results more transparently to the developer
writing the code and behaves similar to other methods that perform
automatic unpack like `#map` or `#each`.

Performance-wise, this change does introduce a speedup on synthetic
benchmarks:

          sort_by:   2.29M (± 1.03%)  1.10× slower
    sort_by (new):   2.51M (± 0.99%)       fastest

Fixes #3419