Consider treating id types as completely opaque

[mzeitlin11]

Is your feature request related to a problem? Please describe.

cc @arlyon, I'd like to propose removing the prefix-based validation of ids in the next branch. Beyond the added complexity and large number of issues (and since there is no documentation of them), it seems like a compatibility hazard given Stripe's statement here

Stripe considers the following changes to be backwards-compatible:
- Changing the length or format of opaque strings, such as object IDs, error messages, and other human-readable strings.
- This includes adding or removing fixed prefixes (such as ch_ on charge IDs).

This potential source of failed requests will be more problematic with miniserde since it won't even provide a useful error indicating the prefix issue.

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