dillon1000/react

[new-architecture] Reassignment-safe frozenness inference

NOTE: this improves the equivalent of "ref kind inference" in the new 
architecture. I'd appreciate review here on the algorithm in particular, but in 
general my plan is to try to implement this on the current architecture. 

The previous InferMutability reference kind inference didn't properly handle 
capturing or reassignment combined with control flow. This is a new version 
(i'll clean up to delete InferMutability entirely) that is less ambitious but 
fully accurate (i hope, hence WIP): 

* Annotates all references of frozen variables as frozen. This includes 
following reassignment, so if you do `const x = props.x; foo(x);` we know that 
`x` is frozen because it derived from a frozen value. This even works 
conditionally, so if `x` is conditionally assigned to some value derived from eg 
props, and you later use `x`, that will be marked as frozen even if it could 
have other values at runtime (since it must conservatively assume a frozen value 
flowed in at runtime). 

* Annotates references that may mutate as mutable. 

* Annotates references that are not frozen, but not mutated _at this reference 
site_, as readonly. It's possible that a readonly usage is followed by a mutable 
usage, since we don't yet know the "lifetime" of the mutability. 

The notable difference from the previous attempt is that we do not attempt to 
find the point at which a formerly-mutable values becomes readonly (and 
therefore eligible for caching). That requires pointer analysis, let's discuss 
offline.