emscripten: directly return Option instead of WasmOption #767

Job	Run time
clippy-test	51s
rustfmt-check	9s
android-check	43s
cargo-publish	0s
asmjs-wasm32-test (asmjs-unknown-emscripten)	1m 17s
macos-test	2m 16s
asmjs-wasm32-test (wasm32-unknown-emscripten)	1m 4s
ubuntu-test	34s
wasm32-bindgen-test (wasm32-unknown-unknown)	21s
wasm32-wasi-test (wasm32-wasi)	21s
windows-test (x86_64)	3m 6s
windows-test (i686)	4m 57s
android-apk-build	1m 34s
ios-build	6m 26s
linux-check-and-test-armv7	1m 43s
wasm-beep-build	22s
	25m 44s

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