Rust: merge structurally equal types in bindgen

[chenyan2002]

Fix #1415

Add merge_structurally_equal_types flag to merge structurally equal types.

• We consider all resource, future and stream types as non-equal, even for resource types with the same TypeId. The reason is that we always need to update the resource table when moving around resources, so we cannot use type alias for any types that contain resources.
• In preprocess, collect_equal_types takes O(n^2) time. If performance is a concern, I can define a hash function to bring down the time to O(n).