Instantiate bindings implementation

This updates instantiate to the new bindings system as discussed with @caridy and @matthewrobb.

This is not passing the tests yet, as we need to first create the corresponding implementation in the ES6 Module Loader. The code can be reviewed initially in the mean time.

@arv is there any chance you can assist with working out how to ensure EmptyStatements don't display on as an empty line statement?
Another option is to keep the state on the transformer. For example, set `this.inExport = true` in `transformExportDeclaration` and then you do not need these if cases and can just check inExport in `transformVariableStatement`