dillon1000/react

Codegen is from ReactiveFunction->AST only

Other than BuildReactiveFunction (HIR -> ReactiveFunction), Codegen.ts was the 
only other remaining place that we use HIRTreeVisitor. However, we've already 
switched the compiler to use the new form of codegen, 
CodegenReactiveFunction.ts. This PR extracts the shared code from Codegen.ts 
into the latter, and deletes the unused bits of Codegen.ts which relied on the 
visitor API. 

This now frees us up to merge BuildReactiveFunction and HIRTreeVisitor, removing 
all the complexity of the visitor trait and type params. 

Note that the `ReactiveFunction` data type can represent non-reactive functions. 
So we can still compile non-React code after this change, the pipeline is `AST` 
-> (BuildHIR) -> `HIR` -> (BuildReactiveFunction) -> `ReactiveFunction` -> 
(CodegenReactiveFunction) -> `AST`. Which is the exact sequence I mapped out at 
the start of this project ;-) (happy that worked out!)