Rework wasm32 support #624

Job	Run time
Run test suite for `reqwest-retry` with every feature combination	39s
Rustfmt	4s
Run test suite for `reqwest-tracing` with every feature combination	5m 50s
Run clippy	1m 10s
Run test suite	1m 14s
Run clippy for every feature combination	1h 14m 6s
Run test suite for `reqwest-middleware` with every feature combination	5m 13s
Docs	53s
Publish dry run	59s
Run `cargo check` for `reqwest-middleware` on `wasm32-unknown-unknown`	33s
Spell Check with Typos	6s
	1h 30m 47s

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