Support of +:, in, in super

[sbarzowski]

Also profiling

[sbarzowski]

@sparkprime @lwander

It's a bit different from what happens in C++:

• For in and in super, C++ retrieves all fields and checks if any matches the right one. Here we just do a regular lookup for a field and then check if it's nil.
• +: is not desugared to a regular field. This way we avoid multiple evaluations of the key and object field lookups.

[coveralls]

Coverage increased (+0.07%) to 83.091% when pulling 3ef2f49 on sbarzowski:desugarer into 850575c on google:master.

[sparkprime]

ping me for merge

[sbarzowski]

@sparkprime ping

[coveralls]

Coverage increased (+0.07%) to 83.297% when pulling ba9694d on sbarzowski:desugarer into 93bcf64 on google:master.