All of our public option structs use a combination of non_exhaustive and generated builders to allow for future expansion and convenient construction.  Rather than generated builders, we should just provide a Default impl for those structs so they can be constructed directly by users with a trailing ..Default::default(); this gets us the same benefit, fits more with general Rust patterns, and removes a dependency and a big swath of generated code that's inflating compile time.

Since this is a substantial (if somewhat mechanical) API surface change, we can make the Default impls available in 3.x and tag the builders as deprecated to prepare users.