Fix CI: pin to Rust 1.94.0, fix clippy warnings, fix feature gates #247

Job	Run time
Pass MSRV values to other jobs	4s
native-backend/stable	4m 17s
nostd/thumbv6m-none-eabi/stable	37s
clippy/stable	1m 0s
cross_test/i686-unknown-linux-gnu/stable	1m 45s
cross_test/s390x-unknown-linux-gnu/stable	1m 46s
format/stable	33s
miri	0s
docs/nightly	0s
cargo-careful	0s
tests/stable	5m 22s
blas-msrv	57s
tests/1.94.0	6m 6s
tests/beta	5m 13s
conclusion	0s
	27m 40s

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