Make math/sample functions take Readers for randomness

[Lúcás Meier]

This follows the lead of go/crypto for example, and allows consumers to choose their own source of randomness instead of having to use Go's (although they should, in most cases).

This also allows us to conveniently use the result of a hash, for example when generating challenges in ZK proofs. This use case is a lot cooler, and would actually simplify logic.