Implementation of `fuzz_target!` is overly restrictive

[cole-miller]

The fuzz_target! macro doesn't accept closures that use a nontrivial pattern for the single argument, like |mut data| { ... }:

https://github.com/rust-fuzz/libfuzzer/blob/e07c487220402f5bec2151de2856a7caff0639c6/src/lib.rs#L204