New system for remote / custom / external types

[bendk]

As discussed in #1865, this is how I think we should update our code for remote / custom / external types.

The doc changes are the first commit so that we can agree on how things should work from the user's perspective.

Changes:

• Make remote types a first-class feature.
• Make UDL generate blanket ffi trait impls for all UniFfiTags. Remove the use_udl_* since we don't need them anymore.
• Add use_remote_type! to handle the one case where we do need to forward ffi traits impls to another crate's tag.
• Use a macro to define custom types. This way we have more freedom to change how things are implemented behind the scenes.
• Update the UDL external syntax so that [Extern] can work with any type.

Benefits are:

• UDL and proc-macros will be consistent in their handling of UniFfiTag.
• First-class remote types would enable things like using anyhow::Error in interfaces.
• The custom type macro is easier to use then the current code. It's also easier for us to make changes behind the scenes.
• External types get a little less hacky.
• 3rd party crates can provide built-in UniFFI support and implement the ffi traits for all their consumers.