dillon1000/react

[rust][sema] Handle var/let/const redeclaration

JavaScript has ~sane~ fun rules around where variables can be redeclared or not, 
and which kinds of variables this applies to. Actually the rules are pretty 
straightforward: 

* `var` can be redeclared any number of times. 

* In strict mode, other declarations cannot be redeclared within the same scope. 
This implies that a `var` declaration cannot conflict with these other forms, 
which must take into account hoisting. So you can't have a `var a; let a` in the 
same scope, but you also can't have a `let a` at a scope and then a `var a` 
which will hoist to that same scope.