der: Custom error types in derive macros.

[turbocool3r]

Currently der_derive only provides macros that implement DecodeValue trait with Error associated type set to ::der::Error. It would be nice to have support for custom error types via an attribute as discussed in #1053.

# Current blocker

The problem is der::Reader lacks support for error types in its [sequence](https://docs.rs/der/latest/der/ trait.Reader.html#method.sequence), read_nested, decode and context_specific methods. I think each of these should gain an additional generic parameter for error type.

I have a patch almost ready so if this is ok, I will move on to create a PR.

Update: after some review I see generic parameters are already implemented in my previous PR 😄

[tarcieri]

Seems like we can close this now that #1560 is merged?

[turbocool3r]

Yes, closing it