Aliases/globals have `expr` duplicated in a `Type`, and that sometimes get recompiled

[msullivan]

In particular, it seems that both can get triggered by self._propagate_if_expr_refs, which causes some weird behavior.

[aljazerzen]

Do I understand correctly:

• Alias & Global both have expr: Expression and type: Type fields, and
• Type also has expr: Expression

... so Expression is duplicated, which is bad for performance, schema size and _propagate_if_expr_refs reasons.

In my opinion, the problematic part is Type having expr field.