Crate xtask_fuzz

Helpers when writing fuzzing crates that get invoked via cargo xtask fuzz.

Might end up getting deprecated, if/when https://github.com/rust-fuzz/cargo-fuzz/issues/346 is resolved

Macros

• fuzz_eprintln
  eprintln! that only gets calls when reproducing fuzz test cases locally
• fuzz_target
  Define a fuzz target.

Functions

• init_tracing_if_repro
  Initialize tracing if this is a repro run.
• is_repro
  Check if the XTASK_FUZZ_REPRO env var was set.