wip: move rand_core requirement under std feature flag

[redshiftzero]

I discovered when compiling poseidon377 using decaf377 0.9.0 on thumbv8m.main-none-eabi that the addition of rand_core in #91 caused a build failure. This is because rand_core requires getrandom, which in turn does not support the target thumbv8m.main-none-eabi (more deets here: https://docs.rs/getrandom/latest/getrandom/#unsupported-targets). This PR sidesteps that issue by moving the new methods requiring rand_core under the std feature flag.

[redshiftzero]

Closing this as not needed, the build failure I was hitting was due to feature unification causing rand_core to pull in the default getrandom feature, that build is now succeeding: penumbra-zone/poseidon377#53