dillon1000/react

ForInStatement: specialize inference/types

Replaces the use of `NextIterableOf` in for-in with a new `NextPropertyOf` 
instruction. The key distinction is `for-of` invokes an arbitrary iterator, 
which means a) each iteration may mutate the collection being iterated and b) 
the returned value may be mutable. However, `for-in` invokes a language-level 
mechanism to iterate: simply iterating alone _cannot_ modify the collection, and 
the returned value is known to be a primitive.