Remove dependency on cargo_metadata #296

Job	Run time
miri	51s
test (no rustup)	43s
build (aarch64-unknown-linux-gnu)	1m 22s
build (aarch64-unknown-linux-musl)	1m 18s
build (aarch64-apple-darwin)	1m 42s
build (aarch64-pc-windows-msvc)	3m 1s
build (x86_64-unknown-linux-gnu)	1m 16s
build (x86_64-unknown-linux-musl)	1m 6s
build (x86_64-apple-darwin)	1m 27s
build (x86_64-pc-windows-msvc)	2m 27s
build (x86_64-unknown-freebsd)	2m 14s
test (stable)	53s
test (beta)	54s
test (nightly)	1m 23s
test (nightly, macos-11)	2m 55s
test (nightly, windows-2019)	2m 27s
deny / deny	18s
msrv / msrv	3m 36s
tidy / tidy	58s
	30m 51s

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