Restrict usages of type variables in non-generalized contexts

Type variables can only be used on functions (and in number literals as
a carve-out for now). In all other cases, a type variable takes on a
single, concrete type based on later usages. This check emits errors
when this is violated.

The implementation is to check the rank of a variable after it could be
generalized. If the variable is not generalized but annotated as a type
variable, emit an error.

crates/compiler/can/src/constraint.rs

@@ -915,7 +915,7 @@ pub struct DefTypes {
     pub loc_symbols: Slice<(Symbol, Region)>,
 }
 
-#[derive(Debug, Clone, Copy)]
+#[derive(Debug, Clone, Copy, PartialEq)]
 pub struct Generalizable(pub bool);
 
 #[derive(Debug, Clone)]