Unused and unassigned variable warnings

[kevin1]

These are pretty easy and I think they would be fun / informative to implement

• warning: unused variable 'foo'
• warning: variable 'foo' is never assigned. Consider declaring it as const.

Could keep a count of the number of times a variable is assigned / referenced. Then go through the topmost symbol table every time you are about to lose scope (I am not sure how to do that bit).

Blocked by #110