Add ObjectReference

[yonaskolb]

Now that PBXObject no longer contains a reference, the helper methods that return a PBXObject are hard to work with as you don't know the reference, for example targets(named: String) -> PBXTarget

This can also be evidenced by the new helper functions in #213 that all return a tuple of the reference and the object.

This PR adds a new genenric type ObjectReference that contains the reference and the object. This is also used in XcodeGen to ease the migration to the removal of reference from PBXObject

This is actually a breaking change. I was hoping to get this into 3.0 but I didn't know it was releasing early.

Another addition to the Objects class that could have also helped is the addition of a getReference(object: PBXObject) -> String that would return the reference for a given object using equatable or class identity, but I thought this would be a cleaner approach

This PR also makes PBXObject Equatable

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This change is 

[pepicrft]

Great idea @yonaskolb. Don't forget to generate the Carthage project and update the changelog. It looks good to me. I plan to add the Sourcery feature this week to include it in the release 4.0, that will also include this change. What do you think?

[yonaskolb]

Sounds good.👍
#204 should be included too

[yonaskolb]

Hmm, just thinking. This could either be [String: PBXGroup] or [ObjectReference<PBXGroup>]

[ilyapuchka]

I think array is better here, to make it is easier to inspect all created groups in order that will match their path order. Also I think with dictionary it will be impossible to get the last created group which is likely to be used in other methods as input parameter.