dillon1000/react

Fix variable-resolution hoisting issues

Fixes one category of bugs with const hoisting. The algorithm finds all consts 
that need to be hoisted, then looks through the statements of a block to find 
the first statement which references that const, delaying the emission of the 
HoistedConst instruction until its actually used. To determine if a statement 
references a const we find every identifier in the statement and check if its 
binding is one of the hoisted bindings. 

There's a very small bug here: when we resolve the binding of each identifier, 
we need to resolve it in its own scope. We're currently resolving these 
identifiers agains the outer block statement's scope, which can cause us to 
misattribute identifiers when there is shadowing: 

``` 

const items = props.items.map(x => x); // we scan this statement, resolve 'x' in 
the block statement scope, and mis-attribute it to the outer x. 

const x = 42; // (1) this x is a candidate for hoisting, so the binding is in 
the set of hoisted consts 

```