instant 0.1.12

A partial replacement for std::time::Instant that works on WASM too.

# v0.1.12

## Added 

- Add `SystemTime` which works in both native and WASM environments.

## Modified

- The `now` function is always available now: there is no need to enable the `now` feature any more. The `now` feature
  still exists (but doesn’t do anything) for backwards compatibility.