This feature flag is not part of the public API, but only for
conditionally rendering sub-circuit graphs. Renaming it stops the
feature from being enabled in the workspace when we only want to enable
the `dev-graph` feature flag of `halo2_proofs`, which _is_ part of its
public API.
- The crate module structure from `orchard` has been flattened.
- The book pages we want to include in `halo2` have been moved to their
target location, to avoid any conflicts during the merge.
- Common files that already exist in zcash/halo2 have been removed.