Consider whether `?` should switch from `From::from` to `Into::into` for Rust 2018

Using `into` instead allows for more conversions to be defined, since it has different coherence rules, which all else being equal would be good.  But because of that, there are (per the [IRLO thread](https://internals.rust-lang.org/t/can-try-and-use-the-into-trait-instead-of-from/6714/12?u=scottmcm)) some uses that are inferable today with `from` but not `into`.