dillon1000/react

First-class representation of builtin jsx tags

We previously represented JsxExpressions using builtin tags - `<div>`, `<b>` etc 
- by lowering the tag name to a Primitive with the string name of the tag. 
However, by lowering into an independent value, it was possible that the lowered 
tag name could be grouped into a different memo slot, such that we ended up with 
output like: 

```javascript 

let t0; 

if (c_1) { 

... 

t0 = "div" 

... 

} else { ... } 

return <t0>{children}</t0> 

``` 

This is obviously wrong. It's also wrong to rename `t0` -> `T0`, because React 
treats that as a custom component, not a builtin. The right thing is to 
explicitly model builtin components, which this PR does by making 
`JsxExpression.tag` be a union of Place | BuiltinTag.