Improve store performance for orders with total quantity > 1000

by refactoring Package#add and converting Package#contents to a Set

fixes #5706

```ruby


# Spree 3.0 latest

                           user     system      total        real
1000 Inventory Units:  3.490000   0.060000   3.550000 (  3.732619)
2000 Inventory Units: 10.760000   0.120000  10.880000 ( 11.182196)
3000 Inventory Units: 23.590000   0.210000  23.800000 ( 24.274458)

# commenting out Spree::Stock::Package#add -> #unless find_item(inventory_unit)
# (no comparison is performed)
                           user     system      total        real
1000 Inventory Units:  1.910000   0.070000   1.980000 (  2.165775)
2000 Inventory Units:  3.460000   0.130000   3.590000 (  3.921917)
3000 Inventory Units:  5.400000   0.180000   5.580000 (  6.061517)


# After converting Package.contents to a Set from Array
                           user     system      total        real
1000 Inventory Units:  2.100000   0.070000   2.170000 (  2.376794)
2000 Inventory Units:  3.700000   0.140000   3.840000 (  4.187232)
3000 Inventory Units:  5.620000   0.200000   5.820000 (  6.382836)

```

----
After the change, `#find_item` is only used by `#remove`. Please advise if `#remove` should also receive a better implementation.

Also note the slight change of comparison behaviour between `#find_item` and the `#eql?`. 

Feedback is appreciated!

Yes. I picked it up directly from the [Ruby Docs](http://www.ruby-doc.org/core-2.1.5/Hash.html#class-Hash-label-Hash+Keys). I will add some comment.