dillon1000/react

[RFC] Refine memoization for Array#map with non-mutating callbacks

Improves memoization for cases such as #2409: 

```javascript 

const x = []; 

useEffect(...); 

return <div>{x.map(item => <span>{item}</span>)}</div>; 

``` 

We previously thought that the `x.map(...)` call mutated `x` since its kind was 
Mutable. However, in this case we can determine that the map call cannot mutate 
`x` (or anything else): the lambda does not mutate any free variables and does 
not mutate its arguments. 

This PR adds a new flag to function signatures, used for method calls only, that 
checks for such cases. The idea is that if the receiver is the only thing that 
is mutable — including that there are no args which are function expressions 
which mutate their parameters — then we can infer the effect as a read. See 
tests which confirm that function expressions which capture or mutate their 
params bypass the optimization.