dillon1000/react

Ensure member path assignments memoize independently

Assignment expressions to a member path are a special case because they're the 
only place where a value isn't assigned to a (possibly temporary) variable, 
which is our unit of memoization. #901 demonstrated how this can lead to values 
that can't be independently memoized: 

```javascript 

const x = {a: a} 

x.y = [b, c]; // array recomputed w `x`, even if only `a` changed 

``` 

This PR ensures that assignment expressions where the LHS is a member path lower 
the RHS to a Place. That means the above example is handled as if you wrote: 

```javascript 

const x = {a: a}; 

const tmp1 = [b, c]; 

x.y = tmp1; 

``` 

And we independently memoize the temporary.