Cannot use std without rand dependency

[str4d]

#136 added rand/std to the std feature. This change forces rand to be in the dependency tree when std is enabled, even if the rand feature is disabled. This makes it impossible to use secp256k1 with std but without rand in the dependency tree.

[elichai]

Hmm that's not good.

I'm not sure what solutions rust offer. we might need to default rand to include std and if you want rand you must get std.
Or have a separate rand-std feature :/

[apoelstra]

Frustrating. rust-lang/cargo#3494

I think a separate rand-std feature is the best option.

[elichai]

Can we close? #176