cms builders: infallible methods with `Result<_>` return type

I'm only working with SignedData & SignerInfo, I haven't checked the other builders, but these methods are all infallible, why do they return Result<_>?

• SignedDataBuilder::add_digest_algorithm
• SignedDataBuilder::add_certificate
• SignedDataBuilder::add_crl
• SignerInfoBuilder::new
• SignerInfoBuilder::add_signed_attribute
• SignerInfoBuilder::add_unsigned_attribute

Would you be open to returning the unwrapped types? The current approach strikes me as unnecessary, and creates a lot of boilerplate in my specific use case where I'm using anyhow to catch errors, which relies on the std/core Error trait for ? coercion (not implemented on v0.2, though I can see it in the v0.3 prereleases).

Despite opening a few issues on this repo in a relatively short timeframe, I'd like to emphasise that my experience across several of the RustCrypto crates has been overwhelmingly positive! Everything I've wanted to do has an API/trait, has felt ergonomic, and has worked once it's compiled ❤️

Start by locating the definitions and call sites for SignedDataBuilder and SignerInfoBuilder, including the six named methods. Compare their behavior with the other CMS builders and inspect existing tests for their Result-returning APIs. Done means documenting the affected API surface and reaching agreement on whether and how infallible return types should change.