no_std: fix build

src/beat_detector.rs

@@ -24,7 +24,7 @@ SOFTWARE.
 use crate::EnvelopeInfo;
 use crate::{AudioHistory, EnvelopeIterator};
 use biquad::{Biquad, Coefficients, DirectForm1, ToHertz, Type, Q_BUTTERWORTH_F32};
-use std::fmt::{Debug, Formatter};
+use core::fmt::{Debug, Formatter};
 
 /// Cutoff frequency for the lowpass filter to detect beats.
 const CUTOFF_FREQUENCY_HZ: f32 = 95.0;