Tests

Implement hash_to_curve for secp256k1 #933

Job	Run time
clippy_check	26s
Rustfmt	13s
test-alloc (secp256kfun)	1m 29s
test-alloc (sigma_fun)	47s
test-alloc (ecdsa_fun)	1m 4s
build	32s
test-alloc (schnorr_fun)	1m 23s
test (x86_64-unknown-linux-gnu)	2m 3s
test (armv7-unknown-linux-gnueabihf)	7m 35s
test-minimal (secp256kfun)	1m 23s
doc-build	24s
test-minimal (sigma_fun)	46s
test-nightly	1m 41s
test-minimal (schnorr_fun)	1m 10s
test-minimal (ecdsa_fun)	1m 5s
	22m 1s

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