dillon1000/react

[rfc] Dont convert ArrowFunction to FunctionDecl

This PR changes the way we compile ArrowFunctionExpression to allow compiling 
more cases, such as within `React.forwardRef()`. We no longer convert arrow 
functions to function declarations. Instead, CodeGenerator emits a generic 
`CodegenFunction` type, and `Program.ts` is responsible for converting that to 
the appropriate type. The rule is basically: 

* Retain the original node type by default. Function declaration in, function 
declaration out. Arrow function in, arrow function out. 

* When gating is enabled, we emit a ConditionalExpression instead of creating a 
temporary variable. If the original (and hence compiled) functions are function 
declarations, we force them into FunctionExpressions only here, since we need an 
expression for each branch of the conditional. Then the rules are: 

* If this is a `export function Foo` ie a named export, replace it with a 
variable declaration with the conditional expression as the initializer, and the 
function name as the variable name. 

* Else, just replace the original function node with the conditional. This works 
for all other cases. 

I'm open to feedback but this seems like a pretty robust approach and will allow 
us to support a lot of real-world cases that we didn't yet, so i think we need 
_something_ in this direction.