This adds a flag to `BundleType` that, when set, requires a dummy
outputs to be produced even if no outputs are added to the builder, and
when unset results in standard padding.
Extracted from the `zcash_primitives` changelog. We only include changes
relative to the `zcash_primitives::sapling` module as of 0.13.0; prior
changes never corresponded to published `sapling-crypto` releases.
Source: https://github.com/zcash/librustzcash
Rev: zcash/librustzcash@6acc64e61c
Kris Nuttycombe
Jack Grigg