Add time support for wasm environments

[Fedeparma74]

This allows ldk to work on wasm targets too, where std::time isn't available. The instant crate emulates the behavior of std::time::SystemTime and std::time::Instant by using javascript functions like Date.now() and performance.now() if the build target is wasm. It's just a wrapper over std::time otherwise. This removes the necessity to use the no-std feature when working with wasm.

[benthecarman]

Concept ACK

[TheBlueMatt]

We definitely shouldn't take a dependency here for all targets. As for the time crate here, I assume it relies on wasm_bindgen providing it some hook to call JS? I'm curious as to the performance of that, we probably don't want to use this in the scorer (cc #1789).

In general, this is maybe an indication we should improve our no-std support - ideally there's minimal pain to just switch to no-std and not have to worry about using a third party crate for something that stdlib is supposed to provide.