time::every subscription improvements / information

[azriel91]

Heya, happened to see that the time module is disabled on WASM. Have recently worked in this space, and learnt:

• it's cheap to swap std::time::Instant for instant::Instant with no cost on native, and it works on WASM.
• async-std::stream::interval works on WASM, but performance is much lower natively compared to governor, and the "unstable" feature brings in the rest of async-std, which may be a bit heavy for some users.
• That said, async-std "just works" on both native and WASM when using instant::Instant.
• governor doesn't yet support WASM, but that's pending Feature/34/support wasm boinkor-net/governor#35

I haven't tried tokio's interval implementation, so can't compare that.

I'm looking at using iced, and so when I have the chance I may implement this, though would be waiting for governor to have support first.