use wasm-friendly instant::Instant everywhere

[mrk-its]

It removes conditional compilation directives and uses instant::{Duration, Instant} instead of std::time::{Duration, Instant}
Fixes #881 It should have minimal impact for bevy on native architectures, because instant::Instant is simply type alias to std::time::Instant there.

[cart]

can we export Duration and Instant from bevy_utils and use that everywhere? That will cut down on the boilerplate across all of the projects and enables bevy package consumers to do time things without needing to care about wasm / pulling in the right dependency

[cart]

Actually with the exception of bevy_tasks which should might eventually have non-bevy consumers who care about dep-count (ex: atelier assets). It should consume instant directly