Change request_timer to work with Duration

[luleyleo]

Since the wasm PR I've been thinking about changing request_timer to take Duration instead of Instant, considering that I've never seen it being called in a different way than

ctx.request_timer(Instant::now() + some_duration)

With this it's just

ctx.request_timer(some_duration)

That would allow us to get rid of Instant as part of our API and thus instant::Instant would not be necessary in user code when targeting wasm.

This should also fix the wasm CI

[luleyleo]

Looks like it does indeed fix the pipeline 😄

[xStrom]

Seems like a good idea to me. Also I want to get the CI fixed sooner than later.