Bump MSRV to 1.66 #292

Job	Run time
miri	56s
build (aarch64-unknown-linux-gnu)	1m 46s
build (aarch64-unknown-linux-musl)	2m 11s
build (aarch64-apple-darwin)	2m 7s
build (aarch64-pc-windows-msvc)	4m 15s
build (x86_64-unknown-linux-gnu)	1m 20s
build (x86_64-unknown-linux-musl)	2m 25s
build (x86_64-apple-darwin)	2m 24s
build (x86_64-pc-windows-msvc)	3m 46s
build (x86_64-unknown-freebsd)	2m 19s
test (stable)	1m 30s
test (beta)	49s
test (nightly)	59s
test (nightly, macos-11)	2m 29s
test (nightly, windows-2019)	3m 9s
deny / deny	19s
msrv / msrv	5m 19s
tidy / tidy	1m 14s
	39m 17s

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