Replace Duration::as_secs by Duration::as_secs_f64

[musikid]

Hi!
This commit just replace the cast to f64 with the appropriate function.
Closes #296.

[coveralls]

Coverage increased (+3.7%) to 38.196% when pulling d8cda02 on MusiKid:duration into c111eb4 on heim-rs:master.

[svartalf]

Thank you for your PR!

Unfortunately, it does not really fixes this issue and also breaks current behavior; please compare Duration::as_secs_f64 sources:
https://github.com/rust-lang/rust/blob/e2a2592885539ca97bfb1232669e7519a0c0703b/library/core/src/time.rs#L643-L645

and current master branch HEAD:

heim/heim-process/src/process/cpu_usage.rs

Lines 27 to 31 in 58d7110

	// TODO: Can be replaced with a `delta_time.as_secs_f64()`
	// as soon as https://github.com/rust-lang/rust/issues/54361 will be stable
	const NANOS_PER_SEC: u32 = 1_000_000_000;
	let mut delta_time_secs =
	(delta_time.as_secs() as f64) + (delta_time.as_nanos() as f64) / (NANOS_PER_SEC as f64);

In our case whole following block must be replaced with Duration::as_secs_f64 call, as it is basically a bundled copy of it:

 const NANOS_PER_SEC: u32 = 1_000_000_000; 
 let mut delta_time_secs = 
     (delta_time.as_secs() as f64) + (delta_time.as_nanos() as f64) / (NANOS_PER_SEC as f64); 

[musikid]

Sorry!
I misunderstood what you were excepting to be replaced. I'm going to change this.

[svartalf]

Great, thank you for your help!