Add: Array#intersection method

[Prajjwal]

Closes Feature #16155.

[ 'a', 'b', 'b', 'z' ].intersection([ 'a', 'b', 'c' ])           #=> [ 'a', 'b' ]
[ 'a', 'b', 'b', 'z' ].intersection([ 'a', 'b', 'c' ], [ 'b' ])  #=> [ 'b' ]
[ 'a' ].intersection                                             #=> [ 'a' ]

@nobu is this acceptable?

Implementation is currently based on Array#&, which is elegant but might end up allocating a whole bunch of arrays holding intermediate results. If needed I can implement Array#intersection so it only allocates the result array once, but then I would like to rewrite Array#& in terms of Array#intersection to keep things DRY.

Let me know what you think.

[connorshea]

This is awesome, thank you!