Bug ? circuit_builder timing | features misconfiguration

[NikitaMasych]

Hey guys!

In plonky2/src/plonk/circuit_builder.rs there is enabled import then feature timing is activated:

#[cfg(feature = "timing")]
use web_time::Instant;

after that, there is something following:

#[cfg(feature = "std")]
let start = Instant::now();

This breaks compilation, if try to add plonky2 with only std feature:

[dependencies]
plonky2 = { version = "1.0.0", default-features = false, features = ["std"] }

since rust can't find Instant type (being unimported because I didn't enable timing)

I guess at the second code snippet shall be conditional for timing and not std, like:

#[cfg(feature = "timing")]
let start = Instant::now();

or maybe conditional import from std, if timing is disabled