Move away from `snafu` for `pub` interfaces

[cbgbt]

Per a comment from @webern on #31:

We are exposing a library and its traits in our public interface and it might be impossible to avoid a major version break if the Snafu library changes in some way.

We may wish to more closely control the fate of our public API by using our own error types, either across the board or by converting to them at the last moment for any pub methods. The downside to the "JIT"-style conversions is that it's not easy to create a mechanism to check this for us at compile-time.

[webern]

Consider, for example, that a user may be completely broken by two major versions of the Context trait (if they are also using Snafu for their own error types).