Optimize Access.filter to eliminate intermediate list creation

[artkay]

This PR replaces the Enum.filter |> Enum.map chain in Access.filter with a more efficient for comprehension that eliminates unnecessary intermediate list creation, which (I believe) should be a somewhat meaningful improvement for large lists.

The tests for the module pass locally:

$> bin/elixir lib/elixir/test/elixir/access_test.exs
Running ExUnit with seed: 375552, max_cases: 20
Excluding tags: [:re_import, {:windows, true}, {:distributed, true}]

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Finished in 0.1 seconds (0.09s on load, 0.01s async, 0.00s sync)
54 doctests, 41 tests, 0 failures

[josevalim]

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