Type Variables can be both int vars and real vars

[sunjay]

Right now we assume that no types can be both int vars and real vars:

dino/src/tycheck/constraints.rs

Lines 134 to 144 in 78a25b4

	/// Records this type variable as an int var so it can be special-cased in the later stages of
	/// type checking. No variable should be both an int var and a real var.
	pub fn ty_var_is_int(&mut self, ty_var: TyVar) {
	self.int_vars.insert(ty_var);
	}
	/// Records this type variable as an real var so it can be special-cased in the later stages of
	/// type checking. No variable should be both an int var and a real var.
	pub fn ty_var_is_real(&mut self, ty_var: TyVar) {
	self.real_vars.insert(ty_var);
	}

No variable should be both an int var and a real var.

This is not valid though because 12 is both a valid integer literal and a valid real literal. We should go through the code and remove this assumption.

A variable can be both an int var and a real var. In that case, the variable must be in the intersection of the set of valid types for both int and real vars.