Switch from thiserror to displaydoc

[Manishearth]

Part of #812

We currently use thiserror, which works like this:

#[derive(Error)]
enum Foo {
   #[error("something {0} something {1}")]
    One(u8, String),
   #[error("something {0}")]
    Two(String)
}

Unfortunately, thiserror isn't no_std, and while there's discussion for getting it there, it's been stalled for a while with some tricky concerns

The main thing we get out of thiserror is Display impls. The std::error::Error impl is largely unimportant, and in our case we can do a blank impl to proxy to Display if we have Display impls.

A crate that does this just for Display is displaydoc. It's inspired by thiserror, and works like this:

#[derive(Display)]
enum Foo {
    /// something {0} something {1}
    One(u8, String),
    /// something {0}
    Two(String)
}

We should switch.

[Manishearth]

A couple issues that might crop up:

• thiserror gives us #[from], which we'll lose out on. We can write these manually (it's not much work), or write our own custom derive for this
• we use Box<dyn Error> in a bunch of places. These APIs are incompatible with no_std. We can:
  • Use core-error. This keeps us compatible with std::error when using std and otherwise works on no_std
  • Just use dyn Display. I prefer this one: ultimately std::error isn't used that much and the error handling working group is working towards revamping it, so the community doesn't really focus on it as much.

[Manishearth]

I realized that we actually already use doc comments on our errors and they're nicer than the error messages themselves, so I added a feature to displaydoc where we can use an attribute instead when we need to: yaahc/displaydoc#31