`Debug` trait not implemented for 77 types

[drew-abbo]

Please discuss your new feature before implementing it if it's nontrivial. Adding a small method is probably trivial, anything larger than that, maybe not. Note that API stability is paramount.

There are 77 types listed on the docs.rs page that do not implement Debug. This makes storing these types in my own types somewhat of a pain, since my own types can then no longer derive Debug.

Here are all of the types without it:

• codec::audio::Audio
• codec::audio::ChannelLayoutIter
• codec::audio::FormatIter
• codec::audio::RateIter
• codec::codec::Codec
• codec::codec::ProfileIter
• codec::context::Context
• codec::decoder::audio::Audio
• codec::decoder::decoder::Decoder
• codec::decoder::opened::Opened
• codec::decoder::subtitle::Subtitle
• codec::decoder::video::Video
• codec::encoder::audio::Audio
• codec::encoder::audio::Encoder
• codec::encoder::encoder::Encoder
• codec::encoder::subtitle::Encoder
• codec::encoder::subtitle::Subtitle
• codec::encoder::video::Encoder
• codec::encoder::video::Video
• codec::packet::borrow::Borrow
• codec::packet::packet::Packet
• codec::packet::packet::SideDataIter
• codec::packet::side_data::SideData
• codec::parameters::Parameters
• codec::subtitle::Ass
• codec::subtitle::AssMut
• codec::subtitle::Bitmap
• codec::subtitle::BitmapMut
• codec::subtitle::RectIter
• codec::subtitle::RectMutIter
• codec::subtitle::Subtitle
• codec::subtitle::Text
• codec::subtitle::TextMut
• codec::video::FormatIter
• codec::video::RateIter
• codec::video::Video
• device::Info
• device::extensions::DeviceIter
• device::input::AudioIter
• device::input::VideoIter
• device::output::AudioIter
• device::output::VideoIter
• filter::context::Context
• filter::context::Sink
• filter::context::Source
• filter::filter::Filter
• filter::filter::PadIter
• filter::graph::Graph
• filter::graph::Parser
• filter::pad::Pad
• format::chapter::Chapter
• format::chapter::ChapterMut
• format::context::destructor::Destructor
• format::context::input::Input
• format::context::input::PacketIter
• format::context::output::Output
• format::format::Input
• format::format::Output
• format::stream::StreamMut
• software::resampling::context::Context
• software::resampling::context::Definition
• software::scaling::context::Context
• software::scaling::filter::Filter
• software::scaling::vector::Vector
• util::dictionary::Iter
• util::format::pixel::Descriptor
• util::format::sample::Buffer
• util::frame::Frame
• util::frame::side_data::SideData
• util::frame::video::Video
• util::interrupt::Interrupt
• util::log::level::LevelError
• codec::subtitle::Rect
• codec::subtitle::RectMut
• format::context::Context
• format::format::Format