dillon1000/react

[hir] add DeclareContext (2/n)

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In `lower`, we now ensure that all context variables are declared by a 
`DeclareContext` instruction. `DeclareContext` always produces a `let` 
declaration, and `StoreContext` is always a reassign. There are a few reasons we 
need `DeclareContext`: 

- DeclareLocal assumes it is storing to a SSA-fied identifier (which always 
stores an immutable primitive). This does not fit context variables. 

- Without DeclareContext, we need custom logic in some passes to initialize 
identifier / context state (e.g. `MutableRange`, ValueKind, etc) for the 
`StoreContext` that declares the context. 

This PR stack models context variables as concrete identifiers (with references 
to context variables modeled by `Place` referencing the context variable 
identifier). @josephsavona pointed out that this is abusing the notion of 
Identifier/Place, as context variables are essentially interior properties of a 
ContextEnvironment. Since we are not modeling `ContextEnvironment` implicitly or 
explicitly, all inference for context variables is essentially pointer analysis.