Simplify Array#& implementation (follow MRI's algo)
Simplify Array#& implementation (follow MRI's algo)

Performance profile is unaffected by this change:
$ bundle exec rake bench:report
Benchmark                                   Opal1  Opal2
benchmark/bm_array_intersection_numbers.rb  1.342  1.344
benchmark/bm_array_intersection_objects.rb  2.595  2.439
benchmark/bm_array_intersection_strings.rb  0.795  0.787
(Opal1 = master branch, Opal2 = array branch)

opal/opal,kachick/opal,Ajedi32/opal,Ajedi32/opal,fazibear/opal,kachick/opal,gausie/opal,jgaskins/opal,suyesh/opal,Flikofbluelight747/opal,iliabylich/opal,jannishuebl/opal,suyesh/opal,Ajedi32/opal,gausie/opal,iliabylich/opal,Mogztter/opal,jannishuebl/opal,merongivian/opal,opal/opal,kachick/opal,gausie/opal,Mogztter/opal,Flikofbluelight747/opal,Flikofbluelight747/opal,suyesh/opal,opal/opal,wied03/opal,merongivian/opal,jannishuebl/opal,Mogztter/opal,Mogztter/opal,merongivian/opal,wied03/opal,fazibear/opal,fazibear/opal,kachick/opal,wied03/opal,iliabylich/opal,jgaskins/opal,opal/opal,jgaskins/opal