dillon1000/react

[hir] Add Load/StoreContext (1/n)

--- 

This PR adds LoadContext and StoreContext to handle reading and writing to 
context variables. 

A context variable is any variable that is declared within a Forget-compiled 
function and reassigned within a closure. Conceptually, we want to treat these 
variables as attributes of a `EnvironmentContext` variable (as most javascript 
VMs do). 

- context variables currently do not participate in type inference (i.e. we do 
not produce type equations for loads from context variables). In the future, we 
can try typing this as `Phi(assignment1Type, assignment2Type, ...)`. 

- context variables are always treated as `Effect.Mutable`. 

- context variables do not participate in SSA, or certain optimizing passes 
(e.g. dead code elimination, constant propagation, etc). 

There is some still follow ups: 

- From my understanding, we should introduce a `DeclareContext` instruction. 

- currently, declaring a context variable (without initializing it) is broken. 
This is because the declaration lowers to `DeclareLocal`, which assumes it is 
storing to a SSA-fied identifier. 

```js 

let x; 

x = 4; 

() => { x = {}; }; 

``` 

- DeclareContext will also make some initialization logic easier. In this PR, I 
added some hack-y code to handle initializing effects / mutable ranges / other 
inference state for the first StoreContext. 

- Handle or bail on stores to context variables through destructuring assignment 

- 

~~Next PR:~~ 

- ~~Change closures to track reassigned identifiers (to extend mutable range of 
primitives)~~