Remove rand-std feature from unstable

Currently the "unstable" feature (used to guard bench mark code) turns
on the "recovery" and "rand-std" features. The "rand-std" feature is not
needed since it is unused, as can be seen by the following bench runs:

Before applying this patch:

...
test benches::bench_sign_ecdsa               ... bench:      35,454 ns/iter (+/- 1,376)
test benches::bench_verify_ecdsa             ... bench:      44,578 ns/iter (+/- 1,619)
test benches::generate                       ... bench:      26,800 ns/iter (+/- 2,352)
test ecdh::benches::bench_ecdh               ... bench:      51,195 ns/iter (+/- 1,400)
test ecdsa::recovery::benches::bench_recover ... bench:      50,174 ns/iter (+/- 1,572)
test key::benches::bench_pk_ordering         ... bench:       5,748 ns/iter (+/- 492)

test result: ok. 0 passed; 0 failed; 76 ignored; 6 measured; 0 filtered out; finished in 14.52s

After removing "rand-std" feature:
...
test benches::bench_sign_ecdsa               ... bench:      35,510 ns/iter (+/- 1,504)
test benches::bench_verify_ecdsa             ... bench:      42,483 ns/iter (+/- 5,628)
test benches::generate                       ... bench:      26,573 ns/iter (+/- 1,333)
test ecdh::benches::bench_ecdh               ... bench:      50,846 ns/iter (+/- 3,982)
test ecdsa::recovery::benches::bench_recover ... bench:      50,908 ns/iter (+/- 2,775)
test key::benches::bench_pk_ordering         ... bench:       6,002 ns/iter (+/- 463)

test result: ok. 0 passed; 0 failed; 60 ignored; 6 measured; 0 filtered out; finished in 6.52s

Author: tcharding

Cargo.toml

@@ -18,7 +18,7 @@ features = [ "rand", "rand-std", "serde", "bitcoin_hashes", "recovery", "global-
 rustdoc-args = ["--cfg", "docsrs"]
 
 [features]
-unstable = ["recovery", "rand-std"]
+unstable = ["recovery"]
 default = ["std"]
 std = ["alloc", "secp256k1-sys/std"]
 # allow use of Secp256k1::new and related API that requires an allocator