Eliminate redundant variables

[sergei-startsev]

The plugin generates redundant variables for some cases, e.g.:

In

export const foo = 'bar';

Out

const foo = 'bar';
var _foo = foo;
export { _foo as foo };
var _foo2 = _foo;
export function rewire$foo($stub) {
  _foo = $stub;
}
export function restore() {
  _foo = _foo2;
}

Added _foo2 variable is redundant here, _foo can be restored by using original foo:

const foo = 'bar';
var _foo = foo;
export { _foo as foo };
- var _foo2 = _foo;
export function rewire$foo($stub) {
  _foo = $stub;
}
export function restore() {
-  _foo = _foo2;
+  _foo = foo;
}

The PR eliminates redundant variables, it generates variables to capture only if required.