fuzz fails due to `unreachable_patterns` errors

[jakoschiko]

With the current nightly Rust toolchain fuzz doesn't work anymore due to compile errors:

error: unreachable pattern
  --> imap-types/src/arbitrary.rs:33:21
   |
33 |                     Err(_) => Err(arbitrary::Error::IncorrectFormat),
   |                     ^^^^^^
...
66 | impl_arbitrary_try_from! { Capability<'a>, Atom<'a> }
   | ----------------------------------------------------- in this macro invocation                                                
   |
   = note: this pattern matches no values because `Infallible` is uninhabited
   = note: `-D unreachable-patterns` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(unreachable_patterns)]`
   = note: this error originates in the macro `impl_arbitrary_try_from` (in Nightly builds, run with -Z macro-backtrace for more info)
fe.de/"

This seems to be caused by this PR rust-lang/rust#122792. Also see rust-lang/rust#129031.