Features from optional dependencies

In crates that don't use the dep: syntax, optional dependencies automatically become Cargo features.

fluvio-wasm-timer cfg(target_arch = "wasm32") implicit feature
futures-timer cfg(not(target_arch = "wasm32")) implicit feature
smol cfg(not(target_arch = "wasm32")) implicit feature
tokio cfg(not(target_arch = "wasm32")) implicit feature

With time

wasm-timer cfg(target_arch = "wasm32") implicit feature