Please don't pull in wasm dependencies on non-wasm platforms

async-h1 depends on `async-std` with the `unstable` feature; that in turn pulls in wasm dependencies via `futures-timer` then `gloo-timers`. Please consider adjusting the feature flag dependencies to avoid pulling `gloo-timers` and its pile of wasm-specific dependencies on non-wasm platforms.

Start by tracing async-h1's Cargo feature and dependency declarations through async-std, futures-timer, and gloo-timers, focusing on the unstable feature and platform conditions. Verify the dependency graph on non-wasm and wasm targets; done means non-wasm builds no longer pull in the wasm-specific dependencies while wasm support remains available.

Written by the indexing model from the issue text.