Enable the `subtle/const-generics` feature #310

Job	Run time
test (stable)	27s
test (1.85.0)	24s
doc	17s
careful	1m 2s
build (1.85.0, wasm32-unknown-unknown)	24s
build (1.85.0, armv7a-none-eabi)	24s
clippy	17s
build (1.85.0, thumbv7em-none-eabi)	24s
build (stable, wasm32-unknown-unknown)	22s
build (stable, armv7a-none-eabi)	22s
miri (x86_64-unknown-linux-gnu)	39s
build (stable, thumbv7em-none-eabi)	27s
miri (s390x-unknown-linux-gnu)	56s
rustfmt	16s
	6m 41s

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