refactor: replace hardcoded EULER_GAMMA with std::f64::consts::EULER_… #149

Job	Run time
clippy/stable	19s
native-backend/stable	41s
format/stable	19s
nostd/thumbv6m-none-eabi/stable	32s
Pass MSRV values to other jobs	4s
cross_test/s390x-unknown-linux-gnu/stable	1m 35s
cross_test/i686-unknown-linux-gnu/stable	1m 35s
miri	-1s
docs/nightly	-1s
cargo-careful	-1s
blas-msrv	1m 7s
tests/1.64.0	1m 9s
tests/beta	1m 2s
tests/stable	58s
conclusion	-1s
	9m 17s

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