[Project] Add a `shallow_eql?` method which performs a cheap comparison.
[NativeTarget] Make adding a target dependency O(1).

This greatly reduces the amount of time taken for large Podfiles.
Various doc and message fixes.
[Project] Make `#==` the fast shallow comparison method.

Because it's too hard to ensure that nobody will introduce changes into
Xcodeproj that will greatly slow down the project again.

For full data comparisons, use `#eql?` instead.
[rubocop] Disable max class length, it doesn't help.
[Object] Cache an object's plist name, which is used often.
[CHANGELOG] Document optimisations.
Merge pull request #216 from CocoaPods/optimisations

Optimisations