Reject useless feature combinations #486

Job	Run time
build (ubuntu-latest)	2m 1s
build (windows-latest)	2m 30s
build-other (wasm32-unknown-unknown, beta)	55s
test (macos-latest, beta)	40s
build-other (wasm32-unknown-unknown, nightly)	58s
test (macos-latest, nightly)	49s
build-other (wasm32-wasi, beta)	55s
test (ubuntu-latest, beta)	54s
build-other (wasm32-wasi, nightly)	1m 34s
test (ubuntu-latest, nightly)	51s
build-other (x86_64-unknown-redox, beta)	1m 1s
test (windows-latest, beta)	1m 18s
build-other (x86_64-unknown-redox, nightly)	1m 23s
test (windows-latest, nightly)	1m 3s
build-other (x86_64-fortanix-unknown-sgx, nightly)	1m 41s
	18m 33s

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