The Scalar type is stored in memory as an array of bytes. This allows easy
access to the bits of a scalar for Scalar x Point operations, at the cost of
forcing a pack/unpack for Scalar x Scalar. This commit splits the Scalar type
into Scalar (packed) and UnpackedScalar (limbs).
This adds a dependency on the `rand` crate, used to construct an
OS-backed CSPRNG. The implementation in this commit is somewhat
inefficient as it constructs a new OsRng object every time; it might be
better to construct it once. (Seems like a lot of overhead for a few
getrandom(2) calls...)
