ruma-events: Feature gate criterion to avoid compilation in CI

[DevinR528]

edit(jplatte): fixes #106

[jplatte]

To have one #[cfg] less, we could move the compile_error! into the alternative main. We could also move all of the actual criterion stuff into a separate #[cfg]ed module, though I'm not sure that's worth it (this would require moving benches/event_deserialize.rs to benches/event_deserialize/main.rs)

[jplatte]

Looks good, my only concern is the IDE thing you noted in #106.

[DevinR528]

This works and doesn't mess up the IDE

#[cfg(not(feature = "criterion_bench"))]
fn main() {
    // To run the benchmarks the "criterion_bench" feature must be enabled.
    // `cargo bench --features criterion_bench --bench <name of the bench>`
    panic!("Enable the criterion_bench feature to run benchmarks");
}

gives

     Running /home/devinr/aprog/rust/__forks__/ruma/target/release/deps/event_deserialize-8bef32f29737f793
thread 'main' panicked at 'Enable the criterion_bench feature to run benchmarks', ruma-events/benches/event_deserialize.rs:124:5
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: bench failed

A compile_warning! would be nice here it's nightly only rust-lang/rust#54140

[jplatte]

Yeah, let's just panic! if the feature flag is missing :)

[jplatte]

I'm relatively certain this is unnecessary.