Revisit several questions about variables after things are more fleshed out

Specifically, these comments from @martijnthe:

https://github.com/martijnthe/jerryscript/pull/5#discussion_r173842251
https://github.com/martijnthe/jerryscript/pull/5#discussion_r173842543
https://github.com/martijnthe/jerryscript/pull/5#discussion_r173843550
https://github.com/martijnthe/jerryscript/pull/5#discussion_r173844201
