Hi folks,

This PR implements another Ruby 2.6 feature: Add a new #filter alias for #select.

For more information, please see feature #13784 [1].

I believe all the related tests (MRI and ruby/spec) are passing: [2], [3], [4], [5], [6], [7], [8], [9], [10], [11].

For reference, I included a link to the commit introducing the functionality in MRI. [12]

Thanks in advance for any feedback.

1. https://bugs.ruby-lang.org/issues/13784
2. https://github.com/ruby/ruby/blob/v2_6_0_rc1/spec/ruby/core/array/filter_spec.rb
3. https://github.com/ruby/ruby/blob/v2_6_0_rc1/spec/ruby/core/enumerable/filter_spec.rb
4. https://github.com/ruby/ruby/blob/v2_6_0_rc1/spec/ruby/core/hash/filter_spec.rb
5. https://github.com/ruby/ruby/blob/v2_6_0_rc1/spec/ruby/library/set/filter_spec.rb
6. https://github.com/ruby/ruby/blob/v2_6_0_rc1/spec/ruby/library/set/sortedset/filter_spec.rb
7. https://github.com/ruby/ruby/blob/v2_6_0_rc1/test/ruby/test_array.rb#L2384-L2401
8. https://github.com/ruby/ruby/blob/v2_6_0_rc1/test/ruby/test_env.rb#L225-L235
9. https://github.com/ruby/ruby/blob/v2_6_0_rc1/test/ruby/test_env.rb#L269-L282
10. https://github.com/ruby/ruby/blob/v2_6_0_rc1/test/ruby/test_hash.rb#L1056-L1092
11. https://github.com/ruby/ruby/blob/v2_6_0_rc1/test/test_set.rb#L548-L564
12. ruby/ruby@b1a8c64