Rename `Delays::generate()` function (or create multiple options) to make it more obvious what kind of unit it expects

[jstuczyn]

sphinx/src/header/delays.rs

Line 31 in 8424f4b

pub fn generate(number: usize, average_delay: f64) -> Vec<Delay> {

[jstuczyn]

Currently, apart from going into the source code, it isn't obvious that it expects the 'average_delay' to be in seconds. Perhaps the most generic approach would be to make it accept time::Duration ?