Future-proof external crate support #53

Job	Run time
Clippy	20s
Doc	16s
Features /	14s
Cross / i686-unknown-linux-gnu	1m 14s
Features / simdutf8 uuid-1	14s
Format	11s
Cross / i586-unknown-linux-gnu	1m 9s
Miri /	39s
Cross / armv7-unknown-linux-gnueabihf	1m 6s
Miri / --release	35s
Cross / aarch64-unknown-linux-gnu	1m 9s
Test / x86_64-pc-windows-msvc	55s
Toolchain / stable	17s
Cross / thumbv6m-none-eabi	1m 9s
Test / x86_64-pc-windows-msvc --release	56s
Toolchain / stable --release	16s
Toolchain / beta	14s
Toolchain / beta --release	16s
Toolchain / nightly	15s
Toolchain / nightly --release	17s
	11m 42s

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