Support trait objects for source in Error

[JelteF]

If you used boxed dyn Error traits as a source a compile error would
occur:

 --> tests/error/derives_for_enums_with_source.rs:5:17
  |
5 | #[derive(Debug, Error)]
  |                 ^^^^^ doesn't have a size known at compile-time
  |
  = help: the trait `Sized` is not implemented for `(dyn std::error::Error + Send + 'static)`
  = help: the trait `std::error::Error` is implemented for `Box<T>`
  = note: required for `Box<(dyn std::error::Error + Send + 'static)>` to implement `std::error::Error`
  = note: required for the cast from `Box<(dyn std::error::Error + Send + 'static)>` to the object type `dyn std::error::Error`
  = note: this error originates in the derive macro `Error` (in Nightly builds, run with -Z macro-backtrace for more info)

This fixes that by shamelessly "borrowing" a fix from thiserror.

Resolves #122

Checklist

• [ ] Documentation is updated (if required)
• Tests are added/updated (if required)
• CHANGELOG entry is added (if required)