Make autoserialize generate fails with a proper error message

[catamorphism]

Preparing for the removal of match check, I changed autoserialize to generate matches instead of match checks. Currently, the _ case fails with no message. Even though this is an impossible case, it's probably better to fail with a more informative message.

[bstrie]

Is autoserialization the same thing as #[auto_encode]? I see exactly one use of #[auto_serialize] in the codebase, which is a test case.

[emberian]

@nikomatsakis this needs clarification

[nikomatsakis]

The code in question has been removed and refactored many times over. I think it's safe to ust close this bug.