Children types declared before parent ones can have clobbering var declarations without error

[Altoids1]

Kind of a wordy title, sorry, but the gist is that this:

/atom/var/bluh
/datum/var/bluh

currently compiles without any errors, even though the former definition is clobbering the latter.

Oversight of #771 (although this problem existed before that PR)

Putting up an issue instead of just instantly fixing it like the chad I am because it actually is a symptom of a greater issue surrounding how to efficiently handle the fact that variable & type definitions can occur in any order in this language.