Tests

Add Shamir secret sharing helpers for scalar polynomials #892

Job	Run time
Rustfmt	14s
build	29s
clippy_check	20s
test (x86_64-unknown-linux-gnu)	1m 56s
test (armv7-unknown-linux-gnueabihf)	7m 34s
doc-build	19s
test-nightly	54s
test-minimal (secp256kfun)	1m 20s
test-alloc (schnorr_fun)	1m 23s
test-alloc (ecdsa_fun)	1m 6s
test-minimal (ecdsa_fun)	1m 1s
test-minimal (sigma_fun)	44s
test-minimal (schnorr_fun)	1m 9s
test-alloc (sigma_fun)	47s
test-alloc (secp256kfun)	1m 19s
	20m 35s

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