Make Scalar::from_bytes work for Scalar<_, NonZero> #742

Job	Run time
build	20s
test (x86_64-unknown-linux-gnu)	46s
test (armv7-unknown-linux-gnueabihf)	5m 40s
Rustfmt	12s
doc-build	13s
clippy_check	10s
test-alloc (secp256kfun)	47s
test-minimal (secp256kfun)	46s
test-alloc (sigma_fun)	9s
test-alloc (ecdsa_fun)	42s
test-minimal (sigma_fun)	18s
test-alloc (schnorr_fun)	44s
test-minimal (ecdsa_fun)	33s
test-minimal (schnorr_fun)	36s
test-nightly	47s
	12m 43s

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