Add the deterministic math/rand helpers to stdlibs

[moul]

Currently, they're only available from gnovm/tests/imports.go; we should migrate them to stdlibs, except for rand.Seed which sets a global state.

And, list in the PR or in this issue some proofs that those functions are deterministic, like https://github.com/golang/go/blob/e7b04a3e166b47c1591eab13f4a451d939ae8941/src/math/rand/rand.go#L378-L381.