To be clear the `#[inline]` does not hint that inlining is beneficial,
but it does give the compiler the option to inline if the compiler
things it would be beneficial.

This starts adding the `#[inline]` attribute to:

1. `IsVariant`: it's expected that this is often beneficial since its
   body is tiny.
2. `Debug`: This is to stay in line with the `std` implementation of the
   `Debug` derive. rust-lang/rust#117727

It also explicitely doesn't add the attribute to the methods of `Error`,
since those are almost never called in hot code paths.

Fixes #317