ensure name suggestions are legal identifiers

[Rich-Harris]

This fixes a bug exposed by rollup/rollup-plugin-babel#55 – it's possible for a module to export a binding that is a reserved word, and for another module to import that reserved word, without ever writing a syntax error...

// helpers.js
var _typeof = x => typeof x;
export { _typeof as typeof };

// main.js
import * as helpers from './helpers.js';
console.log( helpers.typeof( 1 ) );

(The export from helpers.js might be against spec, I'm not sure. That would be a separate issue if it is.)

This PR ensures that the use of helpers.typeof doesn't cause the var _typeof to be rewritten as var typeof.

[eventualbuddha]

Looks good!

[Victorystick]

@Rich-Harris @eventualbuddha I'm pretty sure the only export name that mustn't be a valid variable name is default.