dillon1000/react

[new-arch][easy] Handle declarations (let/const) through codegen

Distinguishes between `LValue` and `Place`. For the most part this is the same 
data structure (LValue composes Place), but it's helpful to distinguish them 
since LValue has other properties such as the kind of declaration. The 
representations may diverge more in the future. 

This change lets us correctly emit code for variable declarations: previously we 
didn't emit `let` or `const`.