Port `OneOf` views from `xilem_web` (#27) #55

Job	Run time
cargo fmt	13s
cargo doc (windows-latest)	39s
cargo clippy + test (windows-latest)	1m 10s
cargo doc (macos-latest)	2m 23s
cargo clippy + test (macos-latest)	1m 12s
cargo doc (ubuntu-latest)	20s
cargo clippy + test (ubuntu-latest)	42s
	6m 39s

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