Add support for #![no_std]

[ogoffart]

I'm working in a library that should be used on wasm and also on MCU platform with #[no_std]. I thought this crate would be a good fit to also have an Instant that can also work in #[no_std].

This allow to use Instant in no_std crate. Added a Instant::from_duration that can be used to construct an instant from a Duration when not using the Rust std lib

What do you think?