Timing variability in `curve25519-dalek`'s `Scalar29::sub`/`Scalar52::sub`

Timing variability of any kind is problematic when working with potentially secret values such as elliptic curve scalars, and such issues can potentially leak private keys and other secrets. Such a problem was recently discovered in curve25519-dalek. The Scalar29::sub 32-bit and Scalar52::sub...