Add global variables to symbol table

[kevin1]

@el2724 pointed out while looking at #100

[edwadli]

typedefs should happen before top level expressions (globals) are evaluated (because our ast separates typedefs from expressions so we don't know whether a particular expr came before a typedef). However, function calls are allowed to reference these globals as long as the globals happen before the function is called.
Consequently, we have to choose:

• dont allow function calls in typedefs
• allow function calls in typedefs, but fail if the function references a global

[kevin1]

I think we should go with the first one — don't allow function calls in typedefs. If we do 2nd one, it becomes pretty non-obvious why this code is wrong:

fun F = 1
type t = { m = F () }

Also there's no way to enforce "no global variables" (aka "pure functions only") if you can call out to C++ — for example, F could be rand() or some arbitrary C++. Then F could return something different every time you init t! And we have a way to make constructors.

Implementation: it may be easier to enforce this at the grammar level rather than in the type/semantic checker.

[edwadli]

ok we will go with the second first option. note that our sast may still generate function calls in typedefs because some of our expressions internally call stdlib functions.

[edwadli]

meant to say first, not second option lol

[edwadli]

closed by #109