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/unify/src/unify.rs

@@ -356,9 +356,9 @@ fn unify_help<M: MetaCollector>(
         }
     } else {
         let error_context = if mismatches.contains(&Mismatch::TypeNotInRange) {
-            ErrorTypeContext::ExpandRanges
+            ErrorTypeContext::EXPAND_RANGES
         } else {
-            ErrorTypeContext::None
+            ErrorTypeContext::empty()
         };
 
         let type1 = env.var_to_error_type_contextual(var1, error_context, observed_pol);