use enum for extension #20

Job	Run time
build (i686-pc-windows-msvc, debug)	54s
build (i686-pc-windows-msvc, release)	1m 11s
build (x86_64-pc-windows-msvc, debug)	48s
build (x86_64-pc-windows-msvc, release)	1m 17s
build (x86_64-apple-darwin, debug)	53s
build (x86_64-apple-darwin, release)	1m 13s
build (aarch64-apple-darwin, debug)	56s
build (aarch64-apple-darwin, release)	1m 3s
build (x86_64-unknown-linux-gnu, debug)	41s
build (x86_64-unknown-linux-gnu, release)	1m 11s
build (i686-unknown-linux-gnu, debug)	48s
build (i686-unknown-linux-gnu, release)	1m 7s
build (aarch64-unknown-linux-gnu, debug)	49s
build (aarch64-unknown-linux-gnu, release)	1m 6s
build (aarch64-linux-android, debug)	42s
build (aarch64-linux-android, release)	1m 4s
build (armv7-linux-androideabi, debug)	57s
build (armv7-linux-androideabi, release)	1m 8s
build (x86_64-linux-android, debug)	54s
build (x86_64-linux-android, release)	1m 21s
build (i686-linux-android, debug)	58s
build (i686-linux-android, release)	1m 4s
build (aarch64-apple-ios, debug)	42s
build (aarch64-apple-ios, release)	1m 9s
build (wasm32-unknown-unknown, debug)	56s
build (wasm32-unknown-unknown, release)	1m 17s
build (aarch64-apple-ios-sim, macos-latest)	1m 5s
	27m 14s

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