Compact inferred functions

[gldubc]

If we infer an intersection of function types using clauses, we compact the clauses which have the same return type (if possible, that is, if the domain can be unioned).

A domain can be unioned if:

1. it's of arity 1
2. it's of arity n > 2, and all the arguments are the same but one which differs (this one gets unioned then).

PS: those two cases are in the added tests.

[josevalim]

I'd like to do this when we generate the function, instead of the descr, because it means we can optimize when we generate the signature, rather than when the function is captured.